BIOLOGY 

LIBRARY 

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MENDELISM 


THE  MACMILLAN  COMPANY 

NEW  YORK    •    BOSTON   •    CHICAGO 
SAN    FRANCISCO 

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TORONTO 


Gregor   Mendel 


M  E  N  D  E  L  I  S  M 


R.   C.    PUNNETT 

FELLOW   OF   GONVILLE  AND   CAIUS   COLLEGE 
PROFESSOR   OF   BIOLOGY   IN   THE   UNIVERSITY   OF   CAMBRIDGE 


THIRD  EDITION 
ENTIRELY  REWRITTEN  AND    MUCH  ENLARGED 


gorfe 

THE    MACMILLAN    COMPANY 
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BY  THE  MACMILLAN  COMPANY. 


Set  up  and  electrotyped.      Published  May,  1911. 


Nnrfoooft  \3rtss 

J.  8.  Gushing  Co.  —  Berwick  &  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


PREFACE 

A  FEW  years  ago  I  published  a  short  sketch  of 
Mendel's  discovery  in  heredity,  and  of  some  of  the 
recent  experiments  which  had  arisen  from  it.  Since 
then  progress  in  these  studies  has  been  rapid,  and  the 
present  account,  though  bearing  the  same  title,  has  been 
completely  rewritten.  A  number  of  illustrations  have 
been  added,  and  here  I  may  acknowledge  my  indebted- 
ness to  Miss  Wheldale  for  the  two  coloured  plates  of 
sweet  peas,  to  the  Hon.  Walter  Rothschild  for  the  but- 
terflies figured  on  Plate  VI.,  to  Professor  Wood  for 
photographs  of  sheep,  and  to  Dr.  Drinkwater  for  the 
figures  of  human  hands.  To  my  former  publishers  also, 
Messrs.  Bowes  and  Bowes,  I  wish  to  express  my  thanks 
for  the  courtesy  with  which  they  acquiesced  in  my 
desire  that  the  present  edition  should  be  published 
elsewhere. 

As  the  book  is  intended  to  appeal  to  a  wide  audience, 
I  have  not  attempted  to  give  more  experimental  in- 
stances than  were  necessary  to  illustrate  the  story,  nor 
have  I  burdened  it  with  bibliographical  reference.  The 
reader  who  desires  further  information  may  be  referred 
to  Mr.  Bateson's  indispensable  volume  on  Mendel's 


225259 


vi  MENDELISM 

Principles  of  Heredity  (Cambridge,  1909),  where  a  full 
account  of  these  matters  is  readily  accessible.  Neither 
have  I  alluded  to  recent  cytological  work  in  so  far  as  it 
may  bear  upon  our  problems.  Many  of  the  facts  con- 
nected with  the  division  of  the  chromosomes  are  striking 
and  suggestive,  but  while  so  much  difference  of  opinion 
exists  as  to  their  interpretation  they  are  hardly  suited 
for  popular  treatment. 

In  choosing  typical  examples  to  illustrate  the  growth 
of  our  ideas  it  was  natural  that  I  should  give  the  prefer- 
ence to  those  with  which  I  was  most  familiar.  For  this 
reason  the  book  is  in  some  measure  a  record  of  the  work 
accomplished  by  the  Cambridge  School  of  Genetics,  and 
it  is  not  unfair  to  say  that  under  the  leadership  of 
William  Bateson  the  contributions  of  this  school  have 
been  second  to  none.  But  it  should  not  be  forgotten 
that  workers  in  other  European  countries,  and  especially 
in  America,  have  amassed  a  large  and  valuable  body  of 
evidence  with  which  it  is  impossible  to  deal  in  a  small 
volume  of  this  scope. 

It  is  not  long  since  the  English  language  was" -enriched 
by  two  new  words  —  Eugenics  and  Genetics  —  and  their 
similarity  of  origin  has  sometimes  led  to  confusion  be- 
tween them  on  the  part  of  those  who  are  innocent  of 
Greek.  Genetics  is  the  term  applied  to  the  experi- 
mental study  of  heredity  and  variation  in  animals  and 
plants,  and  the  main  concern  of  its  students  is  the 
establishing  of  law  and  order  among  the  phenomena 


PREFACE  vii 

there  encountered.  Eugenics,  on  the  other  hand,  deals 
with  the  improvement  of  the  human  race  under  existing 
conditions  of  law  and  sentiment.  The  Eugenist  has  to 
take  into  account  the  religious  and  social  beliefs  and 
prejudices  of  mankind.  Other  issues  are  involved  be- 
sides the  purely  biological  one,  though  as  time  goes  on 
it  is  coming  to  be  more  clearly  recognised  that  the 
Eugenic  ideal  is  sharply  circumscribed  by  the  facts  of 
heredity  and  variation,  and  by  the  laws  which  govern 
the  transmission  of  qualities  in  living  things.  What 
these  facts,  what  these  laws  are,  in  so  far  as  we  at 
present  know  them,  I  have  endeavoured  to  indicate  in 
the  following  pages;  for  I  feel  convinced  that  if  the 
Eugenist  is  to  achieve  anything  solid  it  is  upon  them 
that  he  must  primarily  build.  Little  enough  material, 
it  is  true,  exists  at  present,  but  that  we  now  see  to  be 
largely  a  question  of  time  and  means.  Whatever  be  the 
outcome,  whatever  the  form  of  the  structure  which  is 
eventually  to  emerge,  we  owe  it  first  of  all  to  Mendel 
that  the  foundations  can  be  well  and  truly  laid. 

R.  C.  P. 

CAMBRIDGE,  March,  1911. 


CONTENTS 

CHAPTER   I 
THE  PROBLEM       

CHAPTER   II 
HISTORICAL   ...  .... 

CHAPTER  III 
MENDEL'S  WORK 

CHAPTER   IV. 
THE  PRESENCE  AND  ABSENCE  THEORY 

CHAPTER  V 
INTERACTION  OF  FACTORS 

CHAPTER  VI 
REVERSION *        *      59 


DOMINANCE    . 


CHAPTER  VII 

68 


x  MENDELISM 

CHAPTER   VIII 

PAGE 

WILD  FORMS  AND  DOMESTIC  VARIETIES     ...  79 

CHAPTER   IX 

REPULSION  AND  COUPLING  OF  FACTORS       ....      88 

CHAPTER  X 
SEX        .        .        . 99 

CHAPTER  XI 
SEX  (continued) -115 

CHAPTER  XII 
INTERMEDIATES     .        .        .        .        .        .        ...     125 

CHAPTER  XIII 
VARIATION  AND  EVOLUTION  .        .        .        .        .        .        .     135 

CHAPTER  XIV 
ECONOMICAL :.....    153 

CHAPTER  XV 
MAN .170 

APPENDIX  .        .  .        .        .        .        .     '  .        .187 

INDEX 191 


ILLUSTRATIONS 

PLATES 

PLATK  PAGE 

Gregor  Mendel Frontispiece 

I.    Rabbits To  face  60 

II.   Sweet  Peas      . "64 

III.  Sheep "78 

IV.  Sweet  Peas .  «        80 

V.   Fowls "107 

VI.   Butterflies "146 

FIGURES   IN   TEXT 

FIG. 

1.  Scheme  of  Inheritance  in  simple  Mendelian  Case      .         .  21 

2.  Feathers  of  Silky  and  Common  Fowl                 .         .  30 

3.  Single  and  Double  Primulas '31 

4.  Fowls1  Combs  .........  32 

5.  Diagram  of  Inheritance  of  Fowls1  Combs  .  37 

6.  Fowls1  Combs 40 

7.  Diagram  of  F2  Generation  resulting  from  Cross  between 

two  White  Sweet  Peas 46 

8.  Diagram  illustrating  9 :  3  :  4  Ratio  in  Mice  52 

9.  Sections  of  Primulas 55 


xii  MENDELISM 

FIG.  PACK 

10.  Small  and  Large-eyed  Primulas         .....  56 

11.  Diagram  illustrating  Reversion  in  Pigeons        ...  67 

12.  Primula  sinensis  x  Primula  stellata         «...  69 

13.  Diagram  illustrating  Cross  between  Dominant  and  Re- 

cessive White  Fowls 74 

14.  Bearded  and  Beardless  Wheat 75 

15.  Feet  of  Fowls 77 

16.  Scheme  of  Inheritance  of  Horns  in  Sheep         ...  78 

17.  Abraxas  grossulariata  and  var.  laeticolor          .         .         .  100 

18.  Scheme  of  Inheritance  in  Abraxas   .         .         .         .         .  102 

19.  Scheme  of  Inheritance  of  Silky  Hen  x  Brown  Leghorn 

Cock 106 

20.  Scheme  of  Inheritance  of  Brown  Leghorn  Hen  x  Silky 

Cock .        .        .        .  106 

2 1 .  Scheme  of  Fj  (ex  Brown  Leghorn  x  Silky  Cock)  crossed 

with  pure  Brown  Leghorn     .         .         .         .         .'        .107 

22.  Scheme  for. Silky  Hen  x  Brown  Leghorn  Cock         .         .  109 

23.  Scheme  for  Brown  Leghorn  Hen  x  Silky  Cock         .         .no 

24.  Diagram  illustrating  Nature  of  Offspring  from  Brown  Leg- 

horn Hen  x  Fj  Cock    .         .         .".         .         .         .in 

25.  Scheme  to  illustrate  Heterozygous  Nature  of  Brown  Leg- 

horn Hen 112 

26.  Scheme  of  Inheritance  of  Colour-blindness       .        .        .  117 

27.  Single  and  Double  Stocks         .         .         .         .                  .  123 

28.  F2  Generation  ex  Silky  Hen  x  Brown  Leghorn  Cock        .  127 

29.  Pedigree  of  Eurasian  Family *  .  130 

30.  Curve  illustrating  Influence  of  Selection   .        .         .        .  159 


ILLUSTRATIONS  xiii 

FIG.  PAGE 

31.  Curve  illustrating  Conception  of  pure  Lines  .  .  .     161 

32.  Brachydactylous  and  Normal  Hands         %  .  .  .171 

33.  Radiograph  of  Brachydactylous  Hand       .  .  .  .172 

34.  Pedigree  of  Brachydactylous  Family          .  .  .  173 

35.  Pedigree  of  Haemophilic  Family 175 


For  although  it  be  a  more  new  and  diffi- 
cult way,  to  find  out  the  nature  of  things,  by 
the  things  themfelves ;  then  by  reading  of 
Books,  to  take  our  knowledge  upon  truft 
from  the  opinions  of  Philofophers :  yet  muft 
it  needs  be  confeffed,  that  the  former  is 
much  more  open,  and  leffe  fraudulent,  efpe- 
cially  in  the  Secrets  relating  to  Natural 
Philofophy. 

WILLIAM  HARVEY, 
Anatomical  Exercitations,  1653. 


xiv 


CHAPTER  I 

THE   PROBLEM 

A  CURIOUS  thing  in  the  history  of  human  thought 
so  far  as  literature  reveals  it  to  us  is  the  strange  lack 
of  interest  shown  in  one  of  the  most  interesting  of  all 
human  relationships.  Few  if  any  of  the  more  primitive 
peoples  seem  to  have  attempted  to  define  the  part  played 
by  either  parent  in  the  formation  of  the  offspring,  or  to 
have  assigned  peculiar  powers  of  transmission  to  them, 
even  in  the  vaguest  way.  For  ages  man  must  have  been 
more  or  less  consciously  improving  his  domesticated  races 
of  animals  and  plants,  yet  it  is  not  until  the  time  of  Aris- 
totle that  we  have  clear  evidence  of  any  hypothesis  to 
account  for  these  phenomena  of  heredity.  The  pro- 
duction of  offspring  by  man  was  then  held  to  be  similar 
to  the  production  of  a  crop  from  seed.  The  seed  came 
from  the  man,  the  woman  provided  the  soil.  '  This  re- 
mained the  generally  accepted  view  for  many  centuries, 
and  it  was  not  until  the  recognition  of  woman  as  more 
than  a  passive  agent  that  the  physical  basis  of  heredity 
became  establishec^fcliat  recognition  was  effected  by 
the  microscope,  for  only  with  its  advent  was  actual  ob- 


2  .  , , .  ,     MENDELISM  CHAP. 

servation  of  the  minute  sexual  cells  made  possible.  After 
more  than  a  hundred  years  of  conflict  lasting  until  the  end 
of  the  eighteenth  century,  scientific  men  settled  down  to 
the  view  that  each  of  the  sexes  makes  a  definite  material 
contribution  to  the  offspring  produced  by  their  joint 
efforts.  Among  animals  the  female  contributes  the  ovum 
and  the  male  the  spermatozoon ;  among  plants  the  cor- 
responding cells  are  the  ovules  and  pollen  grains. 

As  a  general  rule  it  may  be  stated  that  the  reproductive 
cells  produced  by  the  female  are  relatively  large  and 
without  the  power  of  independent  movement.  In  addi- 
tion to  the  actual  living  substance  which  is  to  take  part 
in  the  formation  of  a  new  individual,  the  ova  are  more  or 
less  heavily  loaded  with  the  yolk  substance  that  is  to  pro- 
vide for  the  nutrition  of  the  developing  embryo  during  the 
early  stages  of  its  existence.  The  size  of  the  ova  varies 
enormously  in  different  animals.  In  birds  and  reptiles 
where  the  contents  of  the  egg  form  the  sole  resources  of 
the  developing  young  they  are  very  large  in  comparison 
with  the  size  of  the  animal  which  lays  them.  In  mam- 
mals, on  the  other  hand,  where  the  young  are  parasitic 
upon  the  mother  during  the  earlier  stages  of  their  growth, 
the  eggs  are  minute  and  only  contain  the  small  amount  of 
yolk  that  enables  them  to  reach  the  stage  at  which  they 
develop  the  processes  for  attaching  themselves  to  the  wall 
of  the  maternal  uterus.  But  whatever  the  differences  in 
the  size  and  appearance  of  the  ova  produced  by  different 


i  THE   PROBLEM  3 

animals,  they  are  all  comparable  in  that  each  is  a  distinct 
and  separate  sexual  cell  which,  as  a  rule,  is  unable  to  de- 
velop into  a  new  individual  of  its  species  unless  it  is 
fertilised  by  union  with  a  sexual  cell  produced  by  the 
male. 

The  male  sexual  cells  are  always  of  microscopic  size 
and  are  produced  in  the  generative  gland  or  testis  in 
exceedingly  large  numbers.  In  addition  to  their  minuter 
size  they  differ  from  the  ova  in  their  power  of  active  move- 
ment. Animals  present  various  mechanisms  by  which 
the  sexual  elements  may  be  brought  into  juxtaposition, 
but  in  all  cases  some  distance  must  be  traversed  in  a  fluid 
or  semifluid  medium  (frequently  within  the  body  of  the 
female  parent)  before  the  necessary  fusion  can  occur.  To 
accomplish  this  latter  end  of  its  journey  the  spermatozoon 
is  endowed  with  some  form  of  motile  apparatus,  and  this 
frequently  takes  the  form  of  a  long  flagellum,  or  whip-like 
process,  by  the  lashing  of  which  the  little  creature  propels 
itself  much  as  a  tadpole  with  its  tail. 

In  plants  as  in  animals  the  female  cells  or  ovules  are 
larger  than  the  pollen  grains,  though  the  disparity  in  size 
is  not  nearly  so  marked.  Still  they  are  always  relatively 
minute  cells  since  the  circumstances  of  their  develop- 
ment as  parasites  upon  the  mother  plant  render  it  unnec- 
essary for  them  to  possess  any  great  supply  of  food  yolk. 
The  ovules  are  found  surrounded  by  maternal  tissue  in  the 
ovary,  but  through  the  stigma  and  down  the  pistil  a  po- 


4  MENDELISM  CHAP. 

tential  passage  is  left  for  the  male  cell.  The  majority  of 
flowers  are  hermaphrodite,  and  in  many  cases  they  are 
also  self-fertilising.  The  anthers  burst  and  the  contained 
pollen  grains  are  then  shed  upon  the  stigma.  When  this 
happens,  the  pollen  cell  slips  through  a  little  hole  in  its 
coat  and  bores  its  way  down  the  pistil  to  reach  an  ovule 
in  the  ovary.  Complete  fusion  occurs,  and  the  minute 
embryo  of  a  new  plant  immediately  results.  But  for 
some  time  it  is  incapable  of  leading  a  separate  existence, 
and,  like  the  embryo  mammal,  it  lives  as  a  parasite  upon 
its  parent.  By  the  parent  it  is  provided  with  a  protective 
wrapping,  the  seed  coat,  and  beneath  this  the  little  em- 
bryo swells  until  it  reaches  a  certain  size,  when  as  a  ripe 
seed  it  severs  its  connection  with  the  maternal  organism. 
It  is  important  to  realise  that  the  seed  of  a  plant  is  not  a 
sexual  cell  but  a  young  individual  which,  except  for  the 
coat  that  it  wears,  belongs  entirely  to  the  next  generation. 
It  is  with  annual  plants  in  some  respects  as  with  many 
butterflies.  During  one  summer  they  are  initiated  by  the 
union  of  two  sexual  cells  and  pass  through  certain  stages 
of  larval  development  —  the  butterfly  as  a  caterpillar, 
the  plant  as  a  parasite  upon  its  mother.  As  the  summer 
draws  to  a  close  each  passes  into  a  resting-stage  against 
the  winter  cold  —  the  butterfly  as  a  pupa  arid  the  plant 
as  a  seed,  with  the  difference  that  while  the  caterpillar 
provides  its  own  coat,  that  of  the  plant  is  provided  by  its 
mother.  With  the  advent  of  spring  both  butterfly  and 


I  THE   PROBLEM  5 

plant  emerge,  become  mature,  and  themselves  ripen  germ 
cells  which  give  rise  to  a  new  generation. 

Whatever  the  details  of  development,  one  cardinal 
fact  is  clear.  Except  for  the  relatively  rare  instances 
of  parthenogenesis  a  new  individual,  whether  plant  or 
animal,  arises  as  the  joint  product  of  two  sexual  cells 
derived  from  individuals  of  different  sexes.  Such  sexual 
cells,  whether  ovules  or  ova,  spermatozoa  or  pollen  grains, 
are  known  by  the  general  term  of  gametes,  or  marrying 
cells,  and  the  individual  formed  by  the  fusion  or  yoking 
together  of  two  gametes  is  spoken  of  as  a  zygote.  Since  a 
zygote  arises  from  the  yoking  together  of  two  separate 
gametes,  the  individual  so  formed  must  be  regarded 
throughout  its  life  as  a  double  structure  in  which  the 
components  brought  in  by  each  of  the  gametes  remain  in- 
timately fused  in  a  form  of  partnership.  But  when  the 
zygote  in  its  turn  comes  to  form  gametes,  the  partnership 
is  broken  and  the  process  is  reversed.  The  component 
parts  of  the  dual  structure  are  resolved,  with  the  formation 
of  a  set  of  single  structures,  the  gametes. 

The  life  cycle  of  a  species  from  among  the  higher  plants 
or  animals  may  be  regarded  as  falling  into  three  periods : 
(i)  a  period  of  isolation  in  the  form  of  gametes,  each  a  liv- 
ing unit  incapable  of  further  development  without  inti- 
mate association  with  another  produced  by  the  opposite 
sex;  (2)  a  period  of  association  in  which  two  gametes 
become  yoked  together  into  a  zygote  and  react  upon  one 


6  MENDELISM  CHAP. 

another  to  give  rise  by  a  process  of  cell  division  to  what  we 
ordinarily  term  an  individual  with  all  its  various  attri- 
butes and  properties;  and  (3)  a  period  of  dissociation 
when  the  single  structured  gametes  separate  out  from 
that  portion  of  the  double  structured  zygote  which 
constitutes  its  generative  gland.  What  is  the  relation 
between  gamete  and  zygote,  between  zygote  and  gamete  ? 
how  are  the  properties  of  the  zygote  represented  in  the 
gamete,  and  in  what  manner  are  they  distributed  from 
the  one  to  the  other  ?  —  these  are  questions  which  serve  to 
indicate  the  nature  of  the  problem  underlying  the  process 
of  heredity. 

Owing  to  their  peculiar  power  of  growth  and  the  rela- 
tively large  size  to  which  they  attain,  many  of  the  proper- 
ties of  zygotes  are  appreciable  by  observation.  The  col- 
our of  an  animal  or  of  a  flower,  the  shape  of  a  seep!,  or  the 
pattern  on  the  wings  of  a  moth  are  all  zygotic  properties, 
and  all  capable  of  direct  estimation.  It  is  otherwise 
with  the  properties  of  gametes.  While  the  difference  be- 
tween a  black  and  a  white  fowl  is  sufficiently  obvious,  no 
one  by  inspection  can  tell  the  difference  between  the  egg 
that  will  hatch  into  a  black  and  that  which  will  hatch 
into  a  white.  Nor  from  a  mass  of  pollen  grains  can  any 
one  to-day  pick  out  those  that  will  produce  white -from 
those  that  will  produce  coloured  flowers.  Nevertheless, 
we  know  that  in  spite  of  apparent  similarity  there  must 
exist  fundamental  differences  among  the  gametes,  even 


I  THE   PROBLEM  7 

among  those  that  spring  from  the  same  individual.  At 
present  our  only  way  of  appreciating  those  differences 
is  to  observe  the  properties  of  the  zygotes  which  they 
form.  And  as  it  takes  two  gametes  to  form  a  zygote,  we 
are  in  the  position  of  attempting  to  decide  the  proper- 
ties of  two  unknowns  from  one  known.  Fortunately  the 
problem  is  not  entirely  one  of  simple  mathematics.  It 
can  be  attacflM.  by  tte  experimental  method,  and  with 
what  measure  of  success  will  appear  in  the  following 
pages. 


CHAPTER  II 

HISTORICAL 

To  Gregor  Mendel,  monk  and  abbot,  belongs  the  credit 
of  founding  the  modern  science  of  heredity.  Through 
him  there  was  brought  into  these  problems  an  entirely 
new  idea,  an  entirely  fresh  conception  of  the  nature  of 
living  things.  Born  in  1822  of  Austro-Silesian  parentage, 
he  early  entered  the  monastery  of  Briinn,  and  there  in  the 
seclusion  of  the  cloister  garden  he  carried  out  with  the 
common  pea  the  series  of  experiments  which  has  since 
become  so  famous.  In  1865  after  eight  years'  work  he 
published  the  results  of  his  experiments  in  the  Proceedings 
of  the  Natural  History  Society  of  Briinn,  in  a  brief  paper 
of  some  forty  pages.  But  brief  as  it  is  the  importance  of 
the  results  and  the  lucidity  of  the  exposition  will  always 
give  it  high  rank  among  the  classics  of  biological  litera- 
ture. For  thirty -five  years  Mendel's  paper  remained 
unknown,  and  it  was  not  until  1900  that  it  was  simulta- 
neously discovered  by  several  distinguished  botanists. 
The  causes  of  this  curious  neglect  are  not  altogether  with- 
out interest.  Hybridisation  experiments  before  Mendel 
there  had  been  in  plenty.  The  classificatory  work  of  Lin- 


CHAP,  ii  HISTORICAL  9 

naeus  in  the  latter  half  of  the  eighteenth  century  had  given  a 
definite  significance  to  the  word  species,  and  scientific  men 
began  to  turn  their  attention  to  attempting  to  discover 
how  species  were  related  to  one  another.  And  one  ob- 
vious way  of  attacking  the  problem  was  to  cross  different 
species  together  and  see  what  happened.  This  was 
largely  done  during  the  earlier  half  of  the  nineteenth  cen- 
tury, though  such  work  was  almost  entirely  confined  to 
the  botanists.  Apart  from  the  fact  that  plants  lend  them- 
selves to  hybridisation  work  more  readily  than  animals, 
there  was  probably  another  reason  why  zoologists  neg- 
lected this  form  of  investigation.  The  field  of  zoology  is 
a  wider  one  than  that  of  botany,  presenting  a  far  greater 
variety  of  type  and  structure.  Partly  owing  to  their  im- 
portance in  the  study  of  medicine,  and  partly  owing  to 
their  smaller  numbers,  the  anatomy  of  the  vegetable  was 
far  better  known  than  that  of  the  animal  kingdom.  It 
is,  therefore,  not  surprising  that  the  earlier  part  qf^  the 
nineteenth  century  found  the  zoologists,  under  the  in- 
fluence of  Cuvier  and  his  pupils,  devoting  their  entire 
energies  to  describing  the  anatomy  of  the  new  forms  of 
animal  life  which  careful  search  at  home  and  fresh  voyages 
of  discovery  abroad  were  continually  bringing  to  light. 
During  this  period  the  zoologist  had  little  inclination  or 
inducement  to  carry  on  those  investigations  in  hybridisa- 
tion which  were  occupying  the  attention  of  some  botan- 
ists. Nor  did  the  efforts  of  the  botanists  afford  much 


io  MENDELISM  CHAP. 

encouragement  to  such  work,  for  in  spite  of  the  labour 
devoted  to  these  experiments,  the  results  offered  but  a 
confused  tangle  of  facts,  contributing  in  no  apparent  way 
to  the  solution  of  the  problem  for  which  they  had  been 
undertaken.  After  half  a  century  of  experimental  hy- 
bridisation the  determination  of  the  relation  of  species 
and  varieties  to  one  another  seemed  as  remote  as  ever. 
Then  in  1859  came  the  Origin  of  Species,  in  which  Darwin 
presented  to  the  world  a  consistent  theory  to  account 
for  the  manner  in  which  one  species  might  have  arisen 
from  another  by  a  process  of  gradual  evolution.  Briefly 
put,  that  theory  was  as  follows :  In  any  species  of  plant 
or  animal  the  reproductive  capacity  tends  to  outrun  the 
available  food  supply,  and  the  resulting  competition  leads 
to  an  inevitable  struggle  for  existence.  Of  all  the  individ- 
uals born,  only  a  portion,  and  that  often  a  very  small  one, 
can  survive  to  produce  offspring.  According  to  Darwin's 
theory,  the  nature  of  the  surviving  portion  is  not  deter- 
mined by  chance  alone.  No  two  individuals  of  a  species 
are  precisely  alike,  and  among  the  variations  that  occur 
some  enable  their  possessors  to  cope  more  successfully 
with  the  competitive  conditions  under  which  they  exist. 
In  comparison  with  their  less  favoured  brethren  they 
have  a  better  chance  of  surviving  in  the  struggle  for 
existence  and  consequently  of  leaving  offspring.  The 
argument  is  completed  by  the  further  assumption  of  a 
principle  of  heredity,  in  virtue  of  which  offspring  tend  to 


ii  HISTORICAL  ii 

resemble  their  parents  more  than  other  members  of  the 
species.  Parents  possessing  a  favourable  variation  tend 
to  transmit  that  variation  to  their  offspring,  to  some  in 
greater,  to  others  in  less  degree.  Those  possessing  it  in 
greater  degree  will  again  have  a  better  chance  of  survival, 
and  will  transmit  the  favourable  variation  in  even  greater 
degree  to  some  of  their  offspring.  A  competitive  struggle 
for  existence  working  in  combination  with  certain  princi- 
ples of  variation  and  heredity  results  in  a  slow  and  con- 
tinuous transformation  of  species  through  the  operation 
of  a  process  which  Darwin  termed  natural  selection. 

The  coherence  and  simplicity  of  the  theory,  sup- 
ported as  it  was  by  the  great  array  of  facts  which  Darwin 
had  patiently  marshalled  together,  rapidly  gained  the 
enthusiastic  support  of  the  great  majority  of  biologists. 
The  problem  of  the  relation  of  species  at  last  appeared 
to  be  solved,  and  for  the  next  forty  years  zoologists  and 
botanists  were  busily  engaged  in  classifying  by  the  light 
of  Darwin's  theory  the  great  masses  of  anatomical  facts 
which  had  already  accumulated  and  in  adding  and  classi- 
fying fresh  ones.  The  study  of  comparative  anatomy 
and  embryology  received  a  new  stimulus,  for  with  the  ac- 
ceptance of  the  theory  of  descent  with  modification  it  be- 
came incumbent  upon  the  biologist  to  demonstrate  the 
manner  in  which  animals  and  plants  differing  widely  in 
structure*  and  appearance  could  be  conceivably  related 
to  one  another.  Thenceforward  the  energies  of  both 


. 


12  MENDELISM  CHAP. 

botanists  and  zoologists  have  been  devoted  to  the  con- 
struction of  hypothetical  pedigrees  suggesting  the  various 
tracks  of  evolution  by  which  one  group  of  animals  or 
plants  may  have  arisen  from  another  through  a  long  con- 
tinued process  of  natural  selection.  The  result  of  such 
work  on  the  whole  may  be  said  to  have  shown  that  the 
diverse  forms  under  which  living  things  exist  to-day,  and 
have  existed  in  the  past  so  far  as  palaeontology  can  tell 
us,  are  consistent  with  the  view  that  they  are  all  related 
by  the  community  of  descent  which  the  accepted  theory 
of  evolution  demands,  though  as  to  the  exact  course  of 
descent  for  any  particular  group  of  animals  there  is  often 
considerable  diversity  of  opinion.  It  is  obvious  that  all 
this  work  has  little  or  nothing  to  do  with  the  manner  in 
which  species  are  formed.  Indeed,  the  effect  of  Darwin's 
Origin  of  Species  was  to  divert  attention  from  the  way  in 
which  species  originate.  At  the  time  that  it  was  put 
forward  his  explanation  appeared  so  satisfying  that  bi- 
ologists accepted  the  notions  of  variation  and  heredity 
there  set  forth  and  ceased  to  take  any  further  interest  in 
the  work  of  the  hybridisers.  Had  Mendel's  paper  ap- 
peared a  dozen  years  earlier  it  is  difficult  to  believe  that 
it  could  have  failed  to  attract  the  attention  it  deserved. 
Coming  as  it  did  a  few  years  after  the  publication  of  Dar- 
win's great  work,  it  found  men's  minds  set  at  rest  on  the 
problems  that  he  raised  and  their  thoughts  and  energies 
directed  to  other  matters. 


ii  HISTORICAL  13 

Nevertheless  one  interesting  and  noteworthy  attempt 
to  give  greater  precision  to  the  term  heredity  was  made 
about  this  time.  Francis  Galton,  a  cousin  of  Darwin, 
working  upon  data  relating  to  the  breeding  of  Basset 
hounds,  found  that  he  could  express  on  a  definite  statistical 
scheme  the  proportion  in  which  the  different  colours  ap- 
peared in  successive  generations.  Every  individual  was 
conceived  of  as  possessing  a  definite  heritage  which  might 
be  expressed  as  unity.  Of  this,  i  was  on  the  average 
derived  from  the  two  parents  (i.e.  \  from  each  parent), 
}  from  the  four  grandparents,  |  from  the  eight  great- 
grandparents,  and  so  on.  The  Law  of  Ancestral  Heredity, 
as  it  was  termed,  expresses  with  fair  accuracy  some  of  the 
statistical  phenomena  relating  to  the  transmission  of  char- 
acters in  a  mixed  population.  But  the  problem  of  the 
rway  in  which  characters  are  distributed  from  gamete  to 
zygote  and  from  zygote  to  gamete  remained  as  before. 
Heredity  is  essentially  a  physiological  problem,  and 
though  statistics  may  be  suggestive  in  the  initiation  of 
experiment,  it  is  upon  the  basis  of  experimental  fact  that 
progress  must  ultimately  rest.  For  this  reason,  in  spite 
of  its  ingenuity  and  originality,  Galton's  theory  and  the 
subsequent  statistical  work  that  has  been  founded  upon 
it  failed  to  give  us  any  deeper  insight  into  the  nature  of  the 
hereditary  process. 

While  Galton  was  working  in  England  the  German  zool- 
ogist August  Weismann  was  elaborating  the  complicated 


i4  MENDELISM  CHAP. 

theory  of  heredity  which  eventually  appeared  in  his  work 
on  The  Germplasm  (1885),  a  book  which  will  be  remem- 
bered for  one  notable  contribution  to  the  subject.  Until 
the  publication  of  Weismann's  work  it  had  been  generally 
accepted  that  the  modifications  brought  about  in  the 
individual  during  its  lifetime,  through  the  varying 
conditions  of  nutrition  and  environment,  could  be  trans- 
mitted to  the  offspring.  In  this  biologists  were  but  fol- 
lowing Darwin,  who  held  that  the  changes  in  the  parent 
resulting  from  increased  use  or  disuse  of  any  part  or  or- 
gan were  passed  on  to  the  children.  Weismann's  theory 
involved  the  conception  of  a  sharp  cleavage  between  the 
general  body  tissues  or  somatoplasm  and  the  reproductive 
glands  or  germplasm.  The  individual  was  merely  a  car- 
rier for  the  essential  germplasm  whose  properties  had 
been  determined  long  before  he  was  capable  of  leading 
a  separate  existence.  As  this  conception  ran  counter  to 
the  possibility  of  the  inheritance  of  "  acquired  charac- 
ters," Weismann  challenged  the  evidence  upon  which  it 
rested  and  showed  that  it  broke  down  wherever  it  was 
critically  examined.  By  thus  compelling  biologists  to 
revise  their  ideas  as  to  the  inherited  effects  of  use  and  dis- 
use, Weismann  rendered  a  valuable  service  to  the  study 
of  genetics  and  did  much  to  clear  the  way  for  subsequent 
research. 

A  further  important  step  was  taken  in  1895,  when  Bate- 
son  once  more  drew  attention  to  the  problem  of  the  origin 


ii  HISTORICAL  15 

of  species,  and  questioned  whether  the  accepted  ideas  of 
variation  and  heredity  were  after  all  in  consonance  with 
the  facts.  Speaking  generally,  species  do  not  grade  grad- 
ually from  one  to  the  other,  but  the  differences  between 
them  are  sharp  and  specific.  Whence  comes  this  preva- 
lence of  discontinuity  if  the  process  by  which  they  have 
arisen  is  one  of  accumulation  of  minute  and  almost  imper- 
ceptible differences?  Why  are  not  intermediates  of  all 
sorts  more  abundantly  produced  in  nature  than  is  actually 
known  to  be  the  case  ?  Bateson  saw  that  if  we  are  ever 
to  answer  this  question  we  must  have  more  definite  know- 
ledge of  the  nature  of  variation  and  of  the  nature  of  the 
hereditary  process  by  which  these  variations  are  trans- 
mitted. And  the  best  way  to  obtain  that  knowledge  was 
to  let  the  dead  alone  and  to  return  to  the  study  of  the  liv- 
ing. It  was  true  that  the  past  record  of  experimental 
breeding  had  been  mainly  one  of  disappointment.  It 
was  true  also  that  there  was  no  tangible  clue  by  which 
experiments  might  be  directed  in  the  present.  Neverthe- 
less in  this  kind  of  work  alone  there  seemed  any  promise 
of  ultimate  success. 

A  few  years  later  appeared  the  first  volume  of  de  Vries' 
remarkable  book  on  The  Mutation  Theory.  From  a  pro- 
longed study  of  the  evening  primrose  (Oenotherd)  de  Vries 
concluded  that  new  varieties  suddenly  arose  from  older 
ones  by  sudden  sharp  steps  or  mutations,  and  not  by  any 
process  involving  the  gradual  accumulation  of  minute 


16  MENDELISM  CHAP,  n 

differences.  The  number  of  striking  cases  from  among 
widely  different  plants  which  he  was  able  to  bring  forward 
went  far  to  convincing  biologists  that  discontinuity  in 
variation  was  a  more  widespread  phenomenon  than  had 
hitherto  been  suspected,  and  not  a  few  began  to  question 
whether  the  account  of  the  mode  of  evolution  so  generally 
accepted  for  forty  years  was  after  all  the  true  account. 
Such  in  brief  was  the  outlook  in  the  central  problem  of 
biology  at  the  time  of  the  rediscovery  of  Mendel's  work. 


CHAPTER  III 


THE  task  that  Mendel  set  before  himself  was  to  gain 
some  clear  conception  of  the  manner  in  which  the  definite 
and  fixed  varieties  found  within  a  species  are  related  to 
one  another,  and  he  realised  at  the  outset  that  the  best 
chance  of  success  lay  in  working  with  material  of  such  a 
nature  as  to  reduce  the  problem  to  its  simplest  terms.  He 
decided  that  the  plant  with  which  he  was  to  work  must  be 
normally  self-fertilising  and  unlikely  to  be  crossed  through 
the  interference  of  insects,  while  at  the  same  time  it  must 
possess  definite  fixed  varieties  which  bred  true  to  type. 
In  the  common  pea  (Pisum  sativum)  he  found  the  plant 
he  sought.  A  hardy  annual,  prolific,  easily  worked, 
Pisum  has  a  further  advantage  in  that  the  insects  which 
normally  visit  flowers  are  unable  to  gather  pollen  from  it 
and  so  to  bring  about  cross  fertilisation.  At  the  same 
time  it  exists  in  a  number  of  strains  presenting*  well- 
marked  and  fixed  differences.  The  flowers  may  be  purple, 
or  red,  or  white ;  the  plants  may  be  tall  or  dwarf ;  the 
ripe  seeds  may  be  yellow  or  green,  round  or  wrinkled  - 
such  are  a  few  of  the  characters  in  which  the  various  races 
of  peas  differ  from  one  another, 
c  17 


i8  MENDELISM  CHAP. 

In  planning  his  crossing  experiments  Mendel  adopted 
an  attitude  which  marked  him  off  sharply  from  the  earlier 
hybridisers.  He  realised  that  their  failure  to  elucidate 
any  general  principle  of  heredity  from  the  results  of  cross 
fertilisation  was  due  to  their  not  having  concentrated 
upon  particular  characters  or  traced  them  carefully 
through  a  sequence  of  generations.  That  source  of  fail- 
ure he  was  careful  to  avoid,  and  throughout  his  experi- 
ments he  crossed  plants  presenting  sharply  contrasted 
characters,  and  devoted  his  efforts  to  observing  the  be- 
'^haviour  of  these  characters  in  successive  generations. 
Thus  in  one  series  of  experiments  he  concentrated  his  at- 
tention on  the  transmission  of  the  characters  tallness  and 
dwarfness,  neglecting  in  so  far  as  these  experiments  were 
concerned  any  other  characters  in  which  the  parent 
plants  might  differ  from  one  another.  For  this  purpose 
he  chose  two  strains  of  peas,  one  of  about  6  feet  in  height, 
and  another  of  about  ij  feet.  Previous  testing  had 
shown  that  each  strain  bred  true  to  its  peculiar  height. 
These  two  strains  were  artificially  crossed  l  with  one  an- 
other, and  it  was  found  to  make  no  difference  which  was 
used  as  the  pollen  parent  and  which  was  used  as  the  ovule 
parent.  In  either  case  the  result  was  the  same.  The 
result  of  crossing  tall  with  dwarf  was  in  every  case  nothing 
but  tails,  as  tall  or  even  a  little  taller  than  the  tall  parent. 
For  this  reason  Mendel  termed  tallness  the  dominant  and 

1  Cf.  note  on  p.  171. 


in  MENDEL'S   WORK  19 

dwarfness  the  recessive  character.  The  next  stage  was  to 
collect  and  sow  the  seeds  of  these  tall  hybrids.  Such 
seeds  in  the  following  year  gave  rise  to  a  mixed  genera- 
tion consisting  of  tails  and  dwarfs  but  no  intermediates. 
By  raising  a  considerable  number  of  such  plants  Mendel 
was  able  to  establish  the  fact  that  the  jmmhe.r  oLtails 
which  occurred  in  this  generation  was  almost  exactly 
three  times  as  great  as  the  number  of  the  dwarfs.  As  in 
the  previous  year,  seed  were  carefully  collected  from  this, 
the  second  hybrid  generation,  and  in  every  case  the  seeds 
from  each  individual  plant  were  harvested  separately  and 
separately  sown  in  the  following  year.  By  this  respect  for 
the  individuality  of  the  different  plants,  however  closely 
they  resembled  one  another,  Mendel  found  the  clue  that 
had  eluded  the  efforts  of  all  his  predecessors.  The  seeds 
collected  from  the  dwarf  recessives  bred  true,  giving  noth- 
ing but  dwarfs.  And  this  was  true  for  every  dwarf  tested. 
But  with  the  tails  it  was  quite  otherwise.  Although  in- 
distinguishable  in 
appearance,  some  | 

of  them  bred  truQv  T(D) Ft 

while    others    be.- l     I         l 

haved     like     the.     T.          T^D)      ,'  T(D>  Pr--f*  • 

original    tall    hy- 


brids,     giving..     '•    1&»W&KWW**' 

generation       con^,.     T  D — F* 

sisting  'of  tails  and  dwarfs  in  the  proportion  of  three  of 


$0  MENDELISM  CHAP. 

the  former  to  one  of  the  latter.     Counting  showed  that 

»--        .  *       ...  . — •**—  • 

the  number  of  the  tails  which  gave  dwarfs  was  double 
that  of  the  tails  which  bred  true, 

If  we  denote  a  dwarf  plant  as  D,  a  true  breeding  tall 
plant  as  T,  and  a  tall  which  gives  both  tails  and  dwarfs 
in  the  ratio  3 :  i  as  T  (D),  the  result  of  these  experiments 
may  be  briefly  summarised  in  the  foregoing  scheme.1 

Mendel  experimented  with  other  pairs  of  contrasted 
characters  and  found  that  in  every  instance  they  followed 
the  same  scheme  of  inheritance.  Thus  coloured  flowers 
were  dominant  to  white,  in  the  ripe  seeds  yellow  was  dom- 
inant to  green,  and  round  shape  was  dominant  to  wrin- 
kled, and  so  on.  In  every  case  where  the  inheritance  of  an 
alternative  pair  of  characters  was  concerned  the  effect  of 
the  cross  in  successive  generations  was  to  produce  three 
and  only  three  different  sorts  of  individuals,  viz.  domi- 
nants which  bred  true,  dominants  which  gave  both 
dominant  and  recessive  offspring  in  the  ratio  3:1.  and 
recessives  which  always  bred  true.  Having  determined  a 
general  scheme  of  inheritance  which  experiment  showed  to 
hold  good  for  each  of  the  seven  pairs  of  alternative  char- 
acters with  which  he  worked.  Mendel  set  himself  to  pro- 
viding a  theoretical  interpretation  of  this  scheme  which, 
as  he  clearly  realised,  must  be  in  terms  of  germ  cells.  He 

1  It  has  been  found  convenient  to  denote  the  various  generations 
resulting  from  a  cross  by  the  signs  FI.  F:.  F-..  etc.  FI  on  this  system 
denotes  the  first  filial  generation.  F-  the  second  filial  generation  pro- 
duced by  t\vo  parents  belonging  to  the  FI  generation,  and  so  on. 


in 


MENDEL'S   WORK 


21 


conceived  of  the  gametes  as  bearers  of  something  capable 
of  giving  rise  to  the  characters  of  the  plant,  but  he  re- 
garded any  individual  gamete  as  being  able  to  carry  one 
and  one  only  of  any  alternative  pair  of  characters.  A 
given  gamete  could  carry  tallness  or  dwarfness,  but  not 
both.  The  two  were  mutually  exclusive  so  far  as  the 
gamete  was  concerned.  It  must  be  pure  for  one  or  the 
other  of  such  a  pair^and  this  conception  of  the  purity  of 
the  gametes'is  the  most. essential  part  of  Mendel's  theory. 
We  may  now  proceecl  with  the  help  of  the  accompany- 
ing scheme  (Fig.  i)  to  deduce  the  results  that  should  flow 
from  Mendel's  con- 
ception of  the  nature 
of  the  gametes,  and  to  &ametes 
see  how  far  they  are 
in  accordance  with 
the  facts.  Since  the 
original  tall  plant 
belonged  to  a  strain 
which  bred  true,  tall 
the  gametes  produced 
by  it  must  bear  the  tall 
character.  Similarly 
all  the  gametes  of  the 
original  dwarf  plant 
must  bear  the  dwarf 
character.  A  cross  between  these  two  means  the  union  of 


\ 


CL 


Qgametei 


F, 


to 

F2 

generation 
FIG.  i. 

Scheme  of  inheritance  in  the  cross  of  tall  with  dwarf 
pea.  Gametes  represented  by  small  and  zygotes 
by  larger  circles. 


22  MENDELISM  CHAP. 

a  gamete  containing  tallness  with  one  bearing  dwarfness. 
Owing  to  the  completely  dominant  nature  of  the  tall 
character,  such  a  plant  is  in  appearance  indistinguishable 
from  the  pure  tall,  but  it  differs  markedly  from  it  in  the 
nature  of  the  gametes  to  which  it  gives  rise.  When  the 
formation  of  the  gametes  occurs,  the  elements  represent- 
ing dwarfness  and  tallness  segregate  from  one  another,  so 
that  half  of  the  gametes  produced  contain  the  one,  and 
half  contain  the  other  of  these  two  elements.  For  on 
hypothesis  every  gamete  must  be  pure  for  one  or  other  of 
these  two  characters.  And  this  is  true  for  the  ovules  as 
well  as  for  the  pollen  grains.  Such  hybrid  Yl  plants,  there- 
fore, must  produce  a  series  of  ovules  consisting  of  those 
bearing  tallness  and  those  bearing  dwarfness,  and  must 
produce  them  in  equal  numbers.  And  similarly  for  the 
pollen  grains.  We  may  now  calculate  what  should  hap- 
pen when  such  a  series  of  pollen  grains  meets  such  a  series 
of  ovules,  i.e.  the  nature  of  the  generation,  that  should 
be  produced  when  the  hybrid  is  allowed  to  fertilise  itself. 
Let  us  suppose  that  there  are  4  x  ovules  so  that  2  x  are 
"tall"  and  2  x  are  "dwarf."  These  are  brought  in  con  tact 
with  a  mass  of  pollen  grains  of  which  half  are  "tall"  and 
half  are  "dwarf."  It  is  obvious  that  a  "  tall"  ovule  has  an 
equal  chance  of  being  fertilised  by  a  "tall"  or  a  "dwarf" 
pollen  grain.  Hence  of  our  2x  "tall"  ovules,  x  will  be 
fertilised  by  "  tall "  pollen  grains  and  x  will  be  fertilised  by 
"  dwarf  "  pollen  grains.  The  former  must  give  rise  to  tall 


in  MENDEL'S   WORK  23 

plants,  and  since  the  dwarf  character  has  been  entirely 
eliminated  from  them  they  must  in  the  future  breed  true. 
The  latter  must  also  give  rise  to  tall  plants,  but  since  they 
carry  also  the  recessive  dwarf  character  they  must  when 
bred  from  produce  both  tails  and  dwarfs^  Each  of  the  2  x 
dwarf  ovules,  again,  has  an  equal  chance  of  being  fertil- 
ised by  a  "tall"  or  by  a  " dwarf"  pollen  grain.  Hence 
x  will  give  rise  to  tall  plants  carrying  the  recessive  dwarf 
character,  while  x  will  produce  plants  from  which  the  tall 
character  has  been  eliminated,  i.e.  to  pure  recessive  dwarfs. 
Consequently  from  the  4%  ovules  of  the  self- fertilised 
hybrid  we  ought  to  obtain  3  x  tall  and  x  dwarf  plants. 
And  of  the  3  x  tails  x  should  breed  true  to  tallness,  while 
the  remaining  2  x,  having  been  formed  like  the  original 
hybrid  by  the  union  of  a  "tall"  and  a  "dwarf"  gamete, 
ought  to  behave  like  it  when  bred  from  and  give  tails  and 
dwarfs  in  the  ratio  3:1.  Now  this  is  precisely  the  result 
actually  obtained  by  experiment  (cf.  p.  17),  and  the  close 
accord  of  the  experimental  results  with  those  deduced 
on  the  assumption  of  the  purity  of  the  gametes  as  enun- 
ciated by  Mendel  affords  the  strongest  of  arguments  for 
regarding  the  nature  of  the  gametes  and  their  relation  to 
the  characters  of  the  zygotes  in  the  way  that  he  has  done. 
It  is  possible  to  put  the  theory  to  a  further  test.  The 
explanation  of  the  3  :  i  ratio  of  dominants  and  recessives 
in  the  F2  generation  is  regarded  as  due  to  the  Fj  individ- 
uals producing  equal  numbers  of  gametes  bearing  the 


24  MENDELISM  CHAP. 

dominant  and  recessive  elements  respectively.  If  now 
the  F!  plant  be  crossed  with  the  pure  recessive,  we  are 
bringing  together  a  series  of  gametes  consisting  of  equal 
numbers  of  dominants  and  recessives  with  a  series  con- 
sisting solely  of  recessives.  We  ought  from  such  a  cross 
to  obtain  equal  numbers  of  dominant  and  recessive  in- 
dividuals, and  further,  the  dominants  so  produced  ought 
all  to  give  both  dominants  and  recessives  in  the  ratio  3  :  i 
when  they  themselves  are  bred  from.  Both  of  these  ex- 
pectations were  amply  confirmed  by  experiment,  and  cross- 
ing with  the  recessive  is  now  a  recognised  way  of  testing 
whether  a  plant  or  animal  bearing  a  dominant  character 
is  a  pure  dominant,  or  an  impure  dominant  which  is  carry- 
ing the  recessive  character.  In  the  former  case  the  off- 
spring will  be  all  of  the  dominant  form,  while  in  the  latter 
they  will  consist  on  the  average  of  equal  numbers  of 
dominants  and  recessives^ 

So  far  we  have  been  concerned  with  the  results  ob- 
tained when  two  individuals  differing  in  a  single  pair  of 
characters  are  crossed  together  and  with  the  interpreta- 
tion of  those  results.  But  Mendel  also  used  plants 
which  differed  in  more  than  a  single  pair  of  differentiating 
characters.  In  such  cases'  he  found  that  each  pair  of 
characters  followed  the  same  definite  rule,  but  that  the 
inheritance  of  each  pair  was  absolutely  independent  of 
the  other.  Thus,  for  example,  when  a  tall  plant  bear- 
ing coloured  flowers  was  crossed  with  a  dwarf  plant 


in  MENDEL'S   WORK  25 

bearing  white  flowers  the  resulting  hybrid  was  a  tall 
plant  with  coloured  flowers.  For  coloured  flowers  are 
dominant  to  white,  and  tallness  is  dominant  to  dwarf- 
ness.  '  In  the  succeeding  generation  there  are  plants  with 
coloured  flowers  and  plants  with  white  flowers  in  the  pro- 
portion of  3:1,  and  at  the  same  time  tall  plants  and 
dwarf  plants  in  the  same  proportion.  Hence  the  chances 
that  a  tall  plant  will  have  coloured  flowers  are  three 
times  as  great  as  its  chance  of  having  white  flowers. 
And  this  is  also  true  for  the  dwarf  plants.  As  the  result 
of  this  cross,  therefore,  we  should  expect  an  F2  genera- 
tion consisting  of  four  classes,  viz.  coloured  tails,  white 
tails,  coloured  dwarfs,  and  white  dwarfs,  and  we  should 
further  expecjLjLhese  four  forms  to  appear  in  the  ratio  of 
9  coloured  jtalls,.  3  white  tails,  3  coloured  dwarfs,  and  i 
white  dwarf.  For  this  is  the  only  ratio  which  satisfies 
the  conditions  that  the  tails  should  be  to  the  dwarfs  as 
3:1,  and  at  the  same  time  the  coloured  should  be  to 
the  whites"  as  3  :  i .  And  these  are  the  proportions  that 
Mendel  found  to  obtain  actually  in  his  experiments. 
Put  in  a  more  general  form,  it  may  be  stated  that  when 
two  individuals  are  crossed  which  differ  in  two  pairs  of 
differentiating  characters  the  hybrids  (Fj)  are  all  of  the 
same  form,  exhibiting  the  dominant  character  of  each 
of  the  two  pairs r  while  the  F2  generation  produced  by 
such  hybrids  consists  on  the  average  of  9  showing  both 
dominants,  3  showing  one  dominant  and  one  recessive, 


26  MKNDELISM  CHAP. 

3  showing  the  other  dominant  and  the  other  recessive, 
and  i  showing  both  recessive  characters.  And,  as  Men- 
del pointed  out,  the  principle  may  be  extended  in- 
definitely. If,  for  example,  the  parents  differ  in  three 
pair  of  characters  A,  B,  and  C,  respectively  dominant  to 
a,  b,  and  c,  the  Y1  individuals  will  be  all  of  the  form 
ABC,  while  the  F2  generation  will  consists  of  27  ABC, 
9  A  Be;  9  AbC,  9  aBC,  3  Abe,  3  aBc,  3  abC,  and_  i  abc. 
When  individuals  differing  in  a  number  of  alternative 
characters  are  crossed  together,  the  hybrid  generation, 
provided  that  the  original  parents  were  of  pure  strains, 
consists  of  plants  of  the  same  form ;  but  when  these 
are  bred  from  a  redistribution  of  the  various  characters 
occurs.  That  redistribution  follows  the  same  definite 
rule  for  each  character,  and  if  the  constitution  of  the 
original  parents  be  known,  the  nature  of  the  F2  genera- 
tion, i.e.  the  number  of  possible  forms  and  the  propor- 
tions in  which  they  occur,  can  be  readily  calculated. 
Moreover,  as  Mendel  showed,  we  can  calculate  also  the 
chances  of  any  given  form  breeding  true.  To  this  point, 
however,  we  shall  return  later. 

Of  Mendel's  experiments  with  beans  it  is  sufficient  to 
say  here  that  they  corroborated  his  more  ample  work  with 
peas.  He  is  also  known  to  have  made  experiments  with 
many  other  plants,  and  a  few  of  his  results  are  incidentally 
given  in  his  series  of  letters  to  Nageli  the  botanist.  To 
the  breeding  and  crossing  of  bees  he  also  devoted  much 


in  MENDEL'S   WORK  27 

time  and  attention,  but  unhappily  the  record  of  these 
experiments  appears  to  have  been  lost.  The  only  other 
published  work  that  we  possess  dealing  with  heredity  is  a 
brief  paper  on  some  crossing  experiments  with  the  hawk- 
weeds  (Hieracium),  a  genus  that  he  chose  for  working 
with  because  of  the  enormous  number  of  forms  under 
which  it  naturally  exists.  By  crossing  together  the  more 
distinct  varieties,  he  evidently  hoped  to  produce  some  of 
these  numerous  wild  forms,  and  so  throw  light  upon  their 
origin  and  nature.  In  this  hope  he  was  disappointed. 
Owing  in  part  to  the  great  technical  difficulties  attending 
the  cross  fertilisation  of  these  flowers  he  succeeded  in  ob- 
taining very  few  hybrids.  Moreover,  the  behaviour  of 
those  which  he  did  obtain  was  quite  contrary  to  what  he 
had  found  in  t^he  peas.  Instead  of  giving  a  variety  of 
forms  in  the  F2  generation,  they  bred  true  and  continued 
to  do  so  as  long  as  they  were  kept  under  observation. 
More  recent  research  has  shown  that  this  is  due  ta  a  pe- 
culiar form  of  parthenogenesis  (cf.  p.  135),  and  not  to  any 
failure  of  the  characters  to  separate  clearly  from  one  an- 
other in  the  gametes.  Mendel,  however,  could  not  have 
known  of  this,  and  his  inability  to  discover  in  Hieracium 
any  indication  of  the  rule  which  he  had  found  to  hold 
good  for  both  peas  and  beans  must  have  been  a  source 
of  considerable  disappointment.  Whether  for  this 
reason,  or  owing  to  the  utter  neglect  of  his  work  by 
the  scientific  world,  Mendel  gave  up  his  experimental 


28  MEXDELISM  CHAP,  m 

researches  during  the  latter  part  of  his  life.  His 
closing  years  were  shadowed  with  ill-health  and  em- 
bittered by  a  controversy  with  the  Government  on 
a  question  of  the  rights  of  his  monastery.  He  died  of 
B right's  disease  in  1884. 

Note.  —  Shortly  after  the  discovery  of  Mendel's  paper  a  need 
was  felt  for  terms  of  a  general  nature  to  express  the  constitution 
of  individuals  in  respect  ot-inherited  characters,  and  Bateson  ac- 
cordingly proposedkhe  words  homozygote  and  heterozygote.  An 
individual  is  said  to  be  homozygous  for  a  given  character  when  it 
has  been  formed  by  two  gametes  each  bearing  the  character,  and 
all  the  gametes  of  a  homozygote  bear  the  character  in  respect  of 
which  it  is  homozygous.  When,  however,  the  zygote  is  formed  by 
two  gametes  of  which  one  bears  the  given  character  while  the  other 
does  not,  it  is  said  to  be  heterozygous  for  the  character  in  question, 
and  only  half  the  gametes  produced  by  such  a  heterozygote  bear 
the  character.  An  individual  may  be  homozygous  for  one  or  more 
characters,  and  at  the  same  time  may  be  heterozygous  for  others. 


CHAPTER  IV 

THE  PRESENCE  AND  ABSENCE  THEORY 

IT  was  fortunate  for  the  development  of  biological 
science  that  the  rediscovery  of  Mendel's  work  found  a 
small  group  of  biologists  deeply  interested  in  the  problems 
of  heredity,  and  themselves  engaged  in  experimental 
breeding.  To  these  men  the  extraordinary  significance  of 
the  discovery  was  at  once  apparent.  From  their  experi- 
ments, undertaken  in  ignorance  of  Mendel's  paper,  de 
^Vries,  Correns,  and  Tschermak  were  able  to  confirm  his 
results  in  peas  and  other  plants,  while  Bateson  was  the 
first  to  demonstrate  their  application  to  animals.  Thence- 
forward the  record  has  been  one  of  steady  progress,  and 
the  result  of  ten  years'  work  has  been  to  establish  more 
and  more  firmly  the  fundamental  nature  of  MendeFs 
discovery.  The  scheme  of  inheritance,  which  he  was  the 
first  to  enunciate,  has  been  found  to  hold  good  for  such 
diverse  things  as  height,  hairiness,  and  flower  colour  and 
flower  form  in  plants,  the  shape  of  pollen  grains,  and  the 
structure  of  fruits ;  while  among  animals  the  coat  colour 
of  mammals,  the  form  of  the  feathers  and  of  the  comb  in 

poultry,  the  waltzing  habit  of  Japanese  mice,  and  eye 

29 


30  MENDELISM  CHAP. 

colour  in  man  are  but  a  few  examples  of  the  diversity  of 
characters  which  all  follow  the  same  law  of  transmission. 
And  as  time  went  on  many  cases  which  at  first  seemed  to 
fall  without  the  scheme  have  been  gradually  brought  into 
line  in  the  light  of  fuller  knowledge.  Some  of  these  will  be 


FIG.  2. 

A  wing  feather  and  a  contour  feather  of  an  ordinary  and  a  silky  fowl.  The  peculiar 
ragged  appearance  of  the  silky  feathers  is  due  to  the  absence  of  the  little  hooks  or 
barbules  which  hold  the  barbs  together.  The  silky  condition  is  recessive. 

dealt  with  in  the  succeeding  chapters  of  this  book.  Mean- 
while we  may  concern  ourselves  with  the  single  modifica- 
tion of  Mendel's  original  views  which  has  arisen  out  of 
more  ample  knowledge. 

As  we  have  already  seen,  Mendel  considered  that  in  the 
gamete  there  was  either  a  definite  something  correspond- 


iv          PRESENCE   AND   ABSENCE   THEORY         31 

ing  to  the  dominant  character  or  a  definite  something 
corresponding  to  the  recessive  character,  and  that  these 
somethings  whatever  they  were  could  not  coexist  in  any 
single  gamete.  For  these  somethings  we  shall  in  future 
use  the  term  factor.  The  factor,  then,  is  what  corre- 
sponds in  the  gamete  to  the  unit-character  that  appears  in 
some  shape  or  other  in  the  development  of  the  zygote. 
Talmess  in  the  pea  is  a  unit-character,  and  the  gametes  in 


FIG.  3. 

Two  double  and  an  ordinary  single  primula  flower.    This  form  of  double  is  recessive  to 

the  single. 

which  it  is  represented  are  said  to  contain  the  factor  for 
tallness.  Beyond  their  existence  in  the  gamete  and  their 
mode  of  transmission  we  make  no  suggestion  as  to  the 
nature  of  these  factors. 


MENDELISM 


CHAP. 


On  Mendel's  view  there  was  a  factor  corresponding  to 
the  dominant  character  and  another  factor  corresponding 
to  the  recessive  character  of  each  alternative  pair  of  unit- 
characters,  and  the  characters  were  alternative  because  no 


B 


FIG.  4. 

Fowls'  combs.     A,  pea ;  B,  rose ;  C,  single ;  D,  walnut. 

gamete  could  carry  more  than  one  of  the  two  factors 
belonging  to  the  alternative  pair.  On  the  other  hand, 
Mendel  supposed  that  it  always  carried  either  one  or  the 
other  of  such  a  pair.  As  experimental  work  proceeded, 


iv          PRESENCE   AND   ABSENCE  THEORY         33 

it  soon  became  clear  that  there  were  cases  which  could 
not  be  expressed  in  terms  of  this  conception.  The  na- 
ture of  the  difficulty  and  the  way  in  which  it  was  met 
will  perhaps  be  best  understood  by  considering  a  set  of 
experiments  in  which  it  occurred.  Many  of  the  different 
breeds  of  poultry  are  characterised  by  a  particular  form 
of  comb,  and  in  certain  cases  the  inheritance  of  these  has 
been  carefully  worked  out.  It  was  shown  that  the  ro^e 
comb  (Fig.  4,  B)  with  its  flattened  papillated  upper  sur- 
face and  backwardly  projecting  pike  was  dominant  in 
the  ordinary  way  to  the  deeply  serrated  high  single  comb 
(Fig.  4,  C)  which  is  characteristic  of  the  Mediterranean 
races.  Experiment  also  showed  that  the  rjea  comb  (Fig. 
4,  A),  a  form  with  a  low  central  and  two  well-developed 
i lateral  ridges,  such  as^is  found  in  Indian  game,  behaves 
as  a  simple  dominant  to  the  single  -comb.  The  inter- 
esting question  arose  as  to  what  would  happen  when 
the  rose  and  the  pea,  two  forms  each  dominant  to  the 
same  third  form,  were  mated  together.  It  seemed 
reasonable  to  suppose  that  things  which  were  alternative 
to  the  same  thing  would  be  alternative  to  one  another  — 
that  either  rose  or  pea  would  dominate  in  the  hybrids, 
and  that  the  F2  generation  would  consist  of  dominants 
and  recessives  in  the  ratio  3:1.  The  result  of  the  ex- 
periment was,  however,  very  different.  The  cross  rose 
x  pea  led  to  the  production  of  a  comb  quite  unlike  either 
of  them.  This,  the  so-called  walnut  comb  (Fig.  4,  D), 


34  MENDELISM  CHAP. 

from  its  resemblance  to  the  half  of  a  walnut,  is  a  type  of 
comb  which  is  normally  characteristic  of  the  Malay  fowl. 
Moreover,  when  these  Fx  birds  were  bred  together,  a 
further  unlooked-for  result  was  obtained.  As  was  ex- 
pected, there  appeared  in  the  F2  generation  the  three 
forms  walnut,  rose,  and  pea.  But  there  also  appeared 
a  definite  proportion  of  single-combed  birds,  and  among 
many  hundreds  of  chickens  bred  in  this  way  the  propor- 
tions in  which  the  four  forms  walnut,  rose,  pea,  and  single 
appeared  was  9:3:3:1.  Now  this,  as  Mendel  showed,  is 
the  ratio  found  in  an  F2  generation  when  the  original  par- 
ents differ  in  two  pairs  of  alternative  characters,  and  from 
the  proportions  in  which  the  different  forms  of  comb  occur 
Q  j£  ij»  we  must  m^er  that  the  wal- 

Rose     X     Pea        tf     nut    contains   both    domi- 

i 0 i*"*  nants,  the  rose  and  the  pea 

Walnut  x  Walnut  one  dominant  each,  while 

the  single  is  pure  for  both 


Walnut     Rose       Pea      Single  recessive  characters.     This 
(9)          (3)         (3)          (J)     accorded  with   subsequent 

breeding   experiments,    for 

the  singles  bred  perfectly  true  as  soon  as  they  had  once 
made  their  appearance.  So  far  the  case  is  clear.  The 
difficulty  comes  when  we  attempt  to  define  these  two 
pairs  of  characters.  How  are  we  to  express. the  fact  that 
while  single  behaves  as  a  simple  recessive  to  either  pure 
rose,  or  to  pure  pea,  it  can  yet  appear  in  F2  from  a  cross 


iv          PRESENCE   AND   ABSENCE  THEORY         35 

between  these  two  pure  forms,  though  neither  of  them 
should,  on  Mendel's  view,  contain  the  single?  An  ex- 
planation which  covers  the  facts  in  a  simple  way  is  that 
which  has  been  termed  the  "Presence  and  Absence" 
theory.  On  this  theory  the  dominant  character. of  an 
alternative  pair  owes  its  dominance  to  the  presence  oi  SL 
factor  which  is  absent  in  the  recessive.  The  tall  pea  is 
tall  owing  to  the  presence  in  it  .of  the  factor  for  tallness,  but 
in  the  absence  of  this  factor  the  pea  remains  a  dwarf.  All 
peas  are  dwarf ,  but  the  tall  is  a  dwarf  plus  a  factor  which 
turns  it  into  a  tall.  Instead  of  the  characters  of  an  al- 
ternative pair  being  due  to  two  separate  factors,  we  now 
regard  them  as  the  expression  of  the  only  two  possible 
states  of  a  single  factor,  viz.  its  presence  or  its  absence. 
The  conception  will  probably  become  clearer  if  we  follow 
its  application  in  detail  to  the  case  of  the  fowl's  combs. 
In  this  case  we  are  concerned  with  the  transmission  of  the 
two  factors,  rose  (R)  and  pea  (P),  the  presence  of  each  of 
which  is  alternative  to  its  absence.  The  rose-combed 
bird  contains  the  factor  for  rqse^but  not  that  for  pea,  and 
the  pea-combed  bird  contains  the  factor  for  pea  but  not 
that  for  rose.  When  both  factors  are  present  in  a  bird, 
as  in  the  hybrid  made  by  crossing  rose  with  pea,  the  result 
is  a  walnut.  For  convenience  of  argument  we  may  de- 
note the  presence  of  a  given  factor  by  a  capital  letter  and 
its  absence  by  the  corresponding  small  letter.  The  use  of 
the  small  letter  is  merely  a  symbolic  way  of  intimating 


36  MENDELISM  CHAP. 

that  a  particular  factor  is  absent  in  a  gamete  or  zygote. 
Represented  thus  the.zygotic  constitution  of  a  pure  rose- 
combed  bird  is  RRpp;  for  it  has  been  formed  by  the  union 
of  two  gametes  both  of  which  contained  R  but  not  P. 
Similarly  we  may  denote  the  pure  pea-combed  bird  as 
rrPP.  On  crossing  the  rose  with  the  pea  union  occurs 
between  a  gamete  Rp  and  a  gamete  rP,  resulting  in  the 
formation  of  a  heterozygote  of  the  constitution  RrPp. 

•"  «t     » 

The  use  of  the  small  letters  here  informs  us  that  such  a 
zygote  contains  only  a  single  dose  of  each  of  the  factors  R 
and  Pj  although,  of  course,  it  is  possible  for  a  zygote,  if 
made  in  a  suitable  way,  to  have  a  double  dose  of  any 
factor.  Now  when  such  a  bird  comes  to  form  gametes  a 
separation  takes  place  between  the  part  of  the  zygotic  cell 
containing  R  and  the  part  which  does  not  contain  it  (r). 
Half  of  its  gametes,  therefore,  will  contain  R  and  the  other 
half  will  be  without  it  (r).  Similarly  half  of  its  gametes 
will  contain  P  and  the  other  half  will  be  without  it  (p). 
It  is  obvious  that  the  chances  of  R  being  distributed  to  a 
gamete  with  or  without  P  are  equal.  Hence  the  gametes 
containing  R  will  be  of  two  sorts,  PR  and  Rp,  and  these 
will  be  produced  in  equal  numbers.  Similarly  the  gam- 
etes without  R  will  also  be  of  two  sorts,  rP  and  rp,  and 
these,  again,  will  be  produced  in  equal  numbers.  Each  of 
the  hybrid  walnut-combed  birds,  therefore,  gives  rise  to  a 
series  consisting  of  equal  numbers  of  gametes  of  the  four 
different  types  RP,  Rp,  rP,  and  rp ;  and  the  breeding  to- 


IV 


PRESENCE   AND   ABSENCE   THEORY 


37 


gether  of  such  Fx  birds  means  the  bringing  together  of  two 
such  series  of  gametes.  When  this  happens  an  ovum  of 
any  one  of  the  four  types  has  an  equal  chance  of  being 
fertilised  by  a  spermatozoon  of  any  one  of  the  four  types. 
A  convenient  and  simple  method  of  demonstrating  what 
happens  under  such  circumstances  is  the  method  some- 
times termed  the  " chessboard"  method.  For  two  series 
each  consisting  of  four  different  types  of  gamete  we  re- 
quire a  square  divided  up  into  16  parts.  The  four  terms 
of  the  gametic  series  are  first  written  horizontally  across 
the  four  sets  of  four  squares,  so  that  the  series  is  repeated 
four  times.  It  is  then 
written  vertically  four 
times,  care  being 
taken  to  keep  to  the 
same  order.  In  this 
simple  mechanical 
way  all  the  possible 
combinations  are  'rep- 
resented and  in  their 
proper  proportions. 
Fig.  5  shows  the  re- 
sult of  applying  this 
method  to  our  series 
RP,  Rp,  rP, 


RP. 

RP 

RP 
Rp 

RP 
rP 

RP 

H> 

Walnut 

Walnut 

Walnut 

Walnut 

fr 

Rp 
Rp 

RP 

rP 

Rp 

rP 

Walnut 

Rose 

Walnut 

Rose 

rP; 
RP 

rP 
Rp 

rP 
rP 

rP 
rp 

Walnut 

Walnut 

Pea 

Pea 

n>~ 
RP 

2 

Rp 

£ 

rp 
T> 

Walnut 

Rose 

Pea 

Single 

FIG.  5. 

Diagram  to  illustrate  the  nature  of  the  F2  generation 
and  from  the  cross  of  rose  comb  x  pea  comb. 

the  1 6  squares  represent  the  different  kinds  of  zygotes 
formed  and  the  proportions  in  which  they  occur.     As 


38  MENDELISM  CHAP. 

the  figure  shows,  9  zygotes  contain  both  R  and  P, 
having  a  double  or  a  single  dose  of  either  or  both  of 
these  factors.  Such  birds  must  be  all  walnut  combed. 
Three  out  of  the  16  zygotes  contain  R  but  not  P,  and 
these  must  be  rose-combed  birds.  Three,  again,  contain 
P  but  not  R  and  must  be  pea-combed  birds.  Finally 
one  out  of  the  16  contains  neither  R  nor  P.  It  cannot 
be  rose  —  it  cannot  be  pea.  It  must,  therefore,  be  some- 
thing else.  As  a  matter  of  fact  it  is  single.  Why  it 
should  be  single  and  not  something  else  follows  from  what 
we  already  know  about  the  behaviour  of  these  various 
forms  of  comb.  For  rose  is  dominant  to  single  ;  therefore 
on  the  Presence  and  Absence  theory  a  rose  is  a  single  plus 
a  factor  which  turns  the  single  into  a  rose.  If  we  could 
remove  the  "rose"  factor  from  a  rose-combed  bird  the 
underlying  single  would  come  into  view.  Similarly  a  pea 
comb  is  a  single  plus  a  factor  which  turns  the  single  into  a 
pea,  and  a  walnut  is  a  single  which  possesses  two  addi- 
tional modifying  factors.  Singleness,  in  fact,  underlies  all 
these  combs,  and  if  we  write  their  zygotic  constitution  in 
full  we  must  denote  a  walnut  as  RRPPSS,  a  rose  as 
RRppSS,  a  pea  as  rrPPSS,  and  a  single  as  rrppSS.  The 
crossing  of  rose  with  pea  results  in  a  reshuffling  of  the 
factors  concerned,  and  in  accordance  with  the  principle 
of  segregation  some  zygotes  are  formed  in  which  neither 
of  the  modifying  factors  R  and  P  are  present,  and  the 
single  character  can  then  become  manifest. 


iv          PRESENCE   AND   ABSENCE   THEORY         39 

The  Presence  and  Absence  theory  is  to-day  generally 
accepted  by  students  of  these  matters.  Not  only  does  it 
afford  a  simple  explanation  of  the  remarkable  fact  that  in 
all  cases  of  Mendelian  inheritance  we  should  be  able  to 
express  our  unit-characters  in  terms  of  alternative  pairs, 
but,  as  we  shall  have  occasion  to  refer  to  later,  it  suggests 
a  clue  as  to  the  course  by  which  the  various  domesticated 
varieties  of  plants  and  animals  have  arisen  from  their  wild 
prototypes. 

Before  leaving  this  topic  we  may  draw  attention  to 
some  experiments  which  offer  a  pretty  confirmation  of  the 
view  that  the  rose  comb  is  a  single  to  which  a  modifying 
factor  for  roseness  has  been  added.  It  was  argued  that  if 
we  could  find  a  type  of  comb  in  which  the  factor  for  single- 
ness was  absent,  then  on  crossing  such  a  comb  with  a  rose 
we  ought,  if  singleness  really  underlies  rose,  to  obtain 
some  single  combs  in  F2  from  such  a  cross.  Such  a  comb 
we  had  the  good  fortune  to  find  in  the  Breda  fowl,  a  breed 
largely  used  in  Holland.  This  fowl  is  usually  spoken  of  as 
conibless,  for  the  place  of  the  comb  is  taken  by  a  covering 
of  short  bristlelike  feathers  (Fig.  6,  D).  In  reality  it  pos- 
sesses the  vestige  of  a  comb  in  the  form  of  two  minute 
lateral  knobs  of  comb  tissue.  Characteristic  also  of  this 
breed  is  the  high  development  of  the  horny  nostrils,  a 
feature  probably  correlated  with  the  almost  complete 
absence  of  comb.  The  first  step  in  the  experiment  was  to 
prove  the  absence  of  the  factor  for  singleness  in  the  Breda. 


MENDELISM 


CHAP. 


On  crossing  Breda  with  single  the  Yl  birds  exhibit  a  large 
comb  of  the  form  of  a  double  single  comb  in  which  the  two 
portions  are  united  anteriorly,  but  diverge  from  one  an- 
other towards  the  back  of  the  head  (Fig.  6,  C).  The 


FIG.  6. 

Fowls' combs.     A  and  B,  F,  hen  from  rose  x  Breda;  C,  an  Ft  cock  from  the  cross  of 
single  x  Breda;  D,  head  of  Breda  cock. 

Breda  contains  an  element  of  duplicity  which  is  dominant 
to  the  simplicity  of  the  ordinary  single  comb.  But  it  can- 
not contain  the  factor  for  the  single  comb,  because  as  soon 
as  that  is  put  into  it  by  crossing  with  a  single  the  comb 


iv          PRESENCE   AND   ABSENCE   THEORY         41 

assumes  a  large  size,  and  is  totally  distinct  in  appearance 
from  its  almost  complete  absence  in  the  pure  Breda. 
Now  when  the  Breda  is  crossed  with  the  rose  duplicity  is 
dominant  to  simplicity,  and  rose  is  dominant  to  lack  of 
comb,  and  the  F1  generation  consists  of  birds  possessing 
duplex  rose  combs  (Fig.  6,  A  and  B).  On  breeding  such 
birds  together  we  obtain  a  generation  consisting  of 
Bredas,  duplex  roses,  roses,  duplex  singles,  and  singles. 
From  our  previous  experiment  we  know  that  the  singles 

Rose   X  Breda 

I . 


Duplex       x       Duplex 


Rose 


Rose 


r—       — r— 

Duplex       Rose      Duplex     Single       Breda 

Rose  Single  (Duplex 

and  Simplex) 

could  not  have  come  from  the  Breda,  since  a  Breda  comb 
to  which  the  factor  for  single  has  been  added  no  longer 
remains  a  Breda.  Therefore  it  must  have  come  from  the 
rose,  thus  confirming  our  view  that  the  rose  is  in  reality  a 
single  comb  which  contains  in  addition  a  dominant  modi- 
fying factor  (R)  whose  presence  turns  it  into  a  rose.  We 
shall  take  it,  therefore,  that  there  is  good  experimental 
evidence  for  the  Presence  and  Absence  theory,  and  we 
shall  express  in  terms  of  it  the  various  cases  which  come 
up  for  discussion  in  succeeding  chapters. 


CHAPTER  V 

INTERACTION   OF   FACTORS 

WE  have  now  reached  a  point  at  which  it  is  possible  to 
formulate  a  definite  conception  of  the  living  organism.  A 
plant  or  animal  is  a  living  entity  whose  properties  may  in 
large  measure  be  expressed  in  terms  of  unit-characters, 
and  it  is  the  possession  of  a  greater  or  lesser  number  of 
such  unit-characters  renders  it  possible  for  us  to  draw 
**§harp  distinctions  between  one  individual  and  another. 
These  unit-characters  are  represented  by  definite  factors 
in  the  gamete  which  in  the  process  of  heredity  behave  as 
indivisible  entities,  and  are  distributed  according  to  a 
definite  scheme.  The  factor  for  this  or  that  unit-char- 
acter is  either  present  in  the  gamete  or  it  is  riot  present. 
It  must  be  there  in  its  entirety  or  completely  absent. 
Such  at  any  rate  is  the  view  to  which  recent  experiment 
has  led  us.  But  as  to  the  nature  of  these  factors,  the 
conditions  under  which  they  exist  in  the  gamete,  and  the 
manner  in  which  they  produce  their  specific  'effects  in*  the 
zygote,  we  are  at  present  almost  completely  in  the  4ark. 
The  case  of  the  fowls'  combs  opens  up  the  important 

question  of  the  extent  to  which  the  various  factojs  can 

42 


V  INTERACTION  OF   FACTORS  43 

influence  one  another  in  the  zygote.  The  rose  and  the 
pea  factors  are  separate  entities,  and  each  when  present 
alone  produces  a  perfectly  distinct  and  characteristic 
effect  upon  the  single  comb,  turning  it  into  a  rose  or  a  pea 
as  the  case  may  be.  But  when  both  are  present  in  the 
same  zygote  their  combined  effect  is  to  produce  the  wal- 
nut comb,  a  comb  which  is  quite  distinct  from  either  and 
in  no  sense  intermediate  between  them.  The  question 
of  the  influence  of  factors  upon  one  another  did  not  pre- 
sent itself  to  Mendel  because  he  worked  with  characters 
which  affected  different  parts  of  the  plant.  It  was  un- 
likely that  the  factor  which  led  to  the  production  of 
colour  in  the  flower  would  affect  the  shape  of  the  pod,  or 
that  the  height  of  the  plant  would  be  influenced  by 
presence  or  absence  of  the  factor  that  determined  the 
shape  of  the  ripe  seed.  But  when  several  factors  can 
modify  the  same  structure  it  is  reasonable  to  suppose  that 
they  will  influence  one  another  in  the  effects  which  their 
simultaneous  presence  has  upon  the  zygote.  By  them- 
selves the  pea  and  the  rose  factors  each  produce  a  definite 
modification  of  the  single  comb,  but  when  both  are  pres- 
ent in  the  zygote,  whether  as  a  single  or  double  dose,  the 
modification  that  results  is  quite  different  to  that  pro- 
duced by  either  when  present  alone.  Thus  we  are  led 
to  the  conception  of  characters  which  depend  for  their 
manifestation  on  more  than  one  factor  in  the  zygote,  and 
in  the  present  chapter  we  may  consider  a  few  of  the 


44  MENDELISM  CHAP. 

phenomena  which  result  from  such  interaction  between 
separate  and  distinct  factors. 

One  of  the  most  interesting  and  instructive  cases  in 
which  the  interaction  between  separate  factors  has  been 
demonstrated  is  a  case  in  the  sweet  pea.  All  white  sweet 
peas  breed  true  to  whiteness.  And  generally  speaking 
the  result  of  crossing  different  whites  is  to  produce  noth- 
ing but  whites,  whether  in  Fl  or  in  succeeding  generations. 
But  there  are  certain  strains  of .,  white  sweet  peas  which 
when  crossed  together  produce  only  coloured  flowers. 
The  colour  may  be  different  in  different  cases,  though  for 
our  present  purpose  we  may  take  a  case  in  which  the 
colour  is  red.  When  such  reds  are  allowed  to  self- 
fertilise  themselves  in  the  normal  way  and  the  seeds 
White  x  White  sown,  the  resulting  F2  genera- 

|  tion  consists  of  reds  and  whites, 

Red Fj  the  former  being  rather  more 

i ' 1  numerous  than  the  latter  in  the 


White-— F5    proportion  of  g  .  ?>     The  rais. 

ing    of    a    further    generation 

from  the  seeds  of  these  F2  plants  shows  that  the  whites 
always  breed  true  to  whiteness,  but  that  different  reds 
may  behave  differently.  Some  breed  true,  others  give 
reds  and  whites  in  the  ratio  3:1,  while  others,  again,  give 
reds  and  whites  in  the  ratio  9:7.  As  in  the  case  of  the 
fowls'  combs,  this  case  may  be  interpreted  in  terms  of 
the  presence  and  absence  of  two  factors.  Red  in  the 


v  INTERACTION   OF   FACTORS  45 

sweet  pea  results  from  the  interaction  of  two  factors,  and 
unless  these  are  both  present  the  red  colour  cannot  appear. 
Each  of  the  white  parents  carried  one  of  the  two  factors 
whose  interaction  is  necessary  for  the  production  of  the 
red  colour,  and  as  a  cross  between  them  brings  these  two 
complementary  factors  together  the  Fx  plants  must  all  be 
red.  As  this  case  is  of  considerable  importance  for  the 
proper  understanding  of  much  that  is  to  follow,  and  as  it 
has  been  completely  worked  out,  we  shall  consider  it  in 
some  detail.  Denoting  these  two  colour  factors  by  A  and 
B  respectively  we  may  proceed  to  follow  out  the  conse- 
quences of  this  cross.  Since  all  the  Fx  plants  were  red 
the  constitution  of  the  parental  whites  must  have  been 
A  Abb  and  aaBB  respectively,  and  their  gametes  conse- 
quently Ab  and  aB. 

White  White 

The         constitution      AAbb  .      aaBB 

of     the    F!    plants    /   \ 

must,   therefore,  be  Ab          Ab  aB 

AaBb.    Such  a  plant  _  7T 

being    heterozygous  A  B 

for  two  factors  pro-  ^       /    \     w 

duces    a    series    of  g   AB  AB\ 

e    ±1      f  ,&tu  Ab  Ab   cjJtoT 

gametes  of  the  four  yz  aB  aB  *-g 

kinds  AB,  Ab,  aB,          1     ab  a6  J 

ab,     and     produces 

them  in   equal  numbers   (cf.  p.   36).      To  obtain   the 

various  types  of  zygotes  which  are  produced  when  such 


MENDELISM 


CHAP. 


Ab 
Ab 


Ab 
ab 


a  series  of  pollen  grains  meets  a  similar  series  of  ovules 
we  may  make  use  of  the  same  "chessboard"  system 
which  we  have  already  adopted  in  the  case  of  the  fowls' 
combs.  An  examination  of  this  figure  (Fig.  7)  shows 
that  9  out  of  the  16  squares  contain  both  A  and  B,  while  7 
contain  either  A  or  B  .alone,  or  neither.  In  other  words, 
on  this  view  of  the  nature  of  the  two  white  sweet  peas  we 
should  in  the  F2  generation  look  for  the  appearance  of 

coloured  and  white 
flowers  in  the  ratio 
9:7.  And  this,  as 
we  have  already  seen, 
is  what  was  actually 
found  by  experiment. 
Further  examination 
of  the  figure  shows 
that  the  coloured 
plants  are  not  all  of 
the  same  constitution, 
but  are  of  four  kinds 
with  respect  to  their 
zygotic  constitution, 

coloured  Ft. 

viz.   AABB,  AABb, 

AaBB,  and  AaBb.  Since  AABB  is  homozygous  for 
both  A  and  B,  all  the  gametes  which  it  produces  must 
contain  both  of  these  factors,  and  such  a  plant  must 
therefore  breed  true  to  the  red  colour.  A  plant  of  the 


ab 
Ab 


aB 
aB 


ab 
aB 


aB 
ab 


ab 
ab 


FIG.  7. 

Diagram  to  illustrate  the  nature  of  the  F2  generation 
from  the  two  white  sweet  peas  which  give  a 


v  INTERACTION   OF   FACTORS  47 

constitution  AABb  is  homozygous  for  the  factor  A,  but 
heterozygous  for  B.  All  of  its  gametes  will  contain  A ,  but 
only  one-half  of  them  will  contain  B,  i.e.  it  produces  equal 
numbers  of  gametes  AB  and  Ab.  Two  such  series  of 
gametes  coming  together  must  give  a  generation  consist- 
ing of  x  AABB,  2x  AABb,  and  x  AAbb,  that  is,  reds 
and  whites  in  the  ratio  3:1.  Lastly  the  red  zygotes  of 
the  constitution  A  aBb  have  the  same  constitution  as  the 
original  red  made  from  the  two  whites,  and  must  there- 
fore when  bred  from  give  reds  and  whites  in  the  ratio 
9:7.  The  existence  of  all  these  three  sorts  of  reds  was 
demonstrated  by  experiment,  and  the  proportions  in 
which  they  were  met  with  tallied  with  the  theoretical 
explanation. 

The  theory  was  further  tested  by  an  examination  into 
the  properties  of  the  various  F2  whites  which  come  from  a 
coloured  plant  that  has  itself  been  produced  by  the  mating 
of  two  whites.  As  Fig.  7  shows,  these  are,  in  respect  of 
their  constitution,  of  five  different  kinds,  viz.  A  Abb,  Aabb, 
aaBB,  aaBb,  and  aabb.  Since  none  of  them  produce  any- 
thing but  whites  on  self-fertilisation  it  was  found  neces- 
sary to  test  their  properties  in  another  way,  and  the 
method  adopted  was  that  of  crossing  them  together.  It 
is  obvious  that  when  this  is  done  we  should  expect  differ- 
ent results  in  different  cases.  Thus  the  cross  between  two 
whites  of  the  constitution  A  Abb  and  aaBB  should  give 
nothing  but  coloured  plants ;  for  these  two  whites  are  of 


48  MENDELISM  CHAP. 

the  same  constitution  as  the  original  two  whites  from 
which  the  experiment  started.  On  the  other  hand,  the 
cross  between  a  white  of  the  constitution  aabb  and  any 
other  white  can  never  give  anything  but  whites.  For  no 
white  contains  both  A  and  B,  or  it  would  not  be  white, 
and  a  plant  of  the  constitution  aabb  cannot  supply  the 
complementary  factor  necessary  for  the  production  of 
colour.  Again,  two  whites  of  the  constitution  Aabb  and 
aaBb  when  crossed  should  give  both  coloured  and  white 
flowers,  the  latter  being  three  times  as  numerous  as  the 
former.  Without  going  into  further  detail  it  may  be 
stated  that  the  results  of  a  long  series  of  crosses  between 
the  various  F2  whites  accorded  closely  with  the  theoretical 
explanation. 

From  the  evidence  afforded  by  this  exhaustive  set  of 
experiments  it  is  impossible  to  resist  the  deduction  that 
the  appearance  of  colour  in  the  sweet  pea  depends  upon 
the  interaction  of  two  factors  which  are  independently 
transmitted  according  to  the  ordinary  scheme  of  Mende- 
lian  inheritance.  What  these  factors  are  is'still  an  open 
question.  Recent  evidence  of  a  chemical  nature  in- 
dicates that  colour  in  a  flower  is  due  to  the  interaction  of 
two  definitive  substances :  (i)  a  colourless  "chromogen," 
or  colour  basis;  and  (2)  a  ferment  which  behaves  as 
an  activator  of  the  chromogen,  and  by  inducing  some 
process  of  oxidation,  leads  to  the  formation  of  a  coloured 
substance.  But  whether  these  two  bodies  exist  as  such 


v  INTERACTION   OF   FACTORS  49 

in  the  gametes  or  whether  in  some  other  form  we  have  as 
yet  no  means  of  deciding. 

Since  the  elucidation  of  the  nature  of  colour  in  the 
sweet  pea  phenomena  of  a  similar  kind  have  been  wit- 
nessed in  other  plants,  notably  in  stocks,  snapdragons, 
and  orchids.  Nor  is  this  class  of  phenomena  confined  to 
plants.  In  the  course  of  a  series  of  experiments  upon  the 
plumage  colour  of  poultry,  indications  were  obtained 
that  different  white  breeds  did  not  always  owe  their  white- 
ness to  the  same  cause.  Crosses  were  accordingly  made 
between  the  white  Silky  fowl  and  a  pure  white  strain 
derived  from  the  white  Dorking.  Each  of  these  had  been 
previously  shown  to  behave  as  a  simple  recessive  to  colour. 
When  the  two  were  crossed  only  fully  coloured  birds 
resulted.  From  analogy  with  the  case  of  the  sweet  pea 
it  was  anticipated  that  such  Fj  coloured  birds  when  bred 
together  would  produce  an  F2  generation  consisting  of 
coloured  and  white  birds  in  the  ratio  9:7,  and  when  the 
experiment  was  made  this  was  actually  shown  to  be  the 
case.  With  the  growth  of  knowledge  it  is  probable  that 
further  striking  parallels  of  this  nature  between  the  plant 
and  animal  worlds  will  be  met  with. 

Before  quitting  the  subject  of  these  experiments  atten- 
tion may  be  drawn  to  the  fact  that  the  9 :  7  ratio  is  in 
reality  a  9:3:3:1  ratio  in  which  the  last  three  terms 
are  indistinguishable  owing  to  the  special  circumstances 
that  neither  factor  can  produce  a  visible  effect  without 


50  MENDELISM  CHAP. 

the  co-operation  of  the  other.  And  we  may  further  em- 
phasise the  fact  that  although  the  two  factors  thus  inter- 
act upon  one  another  they  are  nevertheless  transmitted 
quite  independently  and  in  accordance  with  the  ordinary 
Mendelian  scheme. 

One  of  the  earliest  sets  of  experiments  demonstrating 
the  interaction  of  separate  factors  was  that  made  by  the 

French  zoologist  Cuenot  on 
Agouti      X     Albino 

I  the  coat  colours  of  mice. 

.    I    !     3      IT1   t  It  was  shown  that  in  cer- 

Agouti     X      Agouti 

tain    cases    agouti,    which 

I 1 1        is     the     colour     of     the 

A?outi  Black  Albino          ,.  .*,  , 

°°}  (3)  (4)       ordinary  wild  grey  mouse, 

behaves  as  a  dominant  to 

the  albino  variety,  i.e.  the  F2  generation  from  such  a  cross 
consists  of  agoutis  and  albinos  in  the  ratio  3:1.  But  in 
other  cases  the  cross  between  albino  and  agouti  gave  a 
different  result.  In  the  FI  generation  appeared  only 
agoutis  as  before,  but  the  F2  generation  consisted  of  three 
distinct  types,  viz.  agoutis,  albinos,  and  blacks.  Whence 
the  sudden  appearance  of  the  new  type  ?  The  answer  is  a 
simple  one.  The  albino  parent  was  really  a  black.  But 
it  lacked  the  factor  without  which  the  colour  is  unable  to 
develop,  and  consequently  it  remained  an  albino.  If  we 
denote  this  factor  by  C,  then  the  constitution  of  an  albino 
must  be  cc,  while  that  of  a  coloured  animal  may  be  CC  or 
Cc,  according  as  to  whether  it  breeds  true  to  colour  or  can 


v  INTERACTION  OF   FACTORS  51 

throw  albinos.  Agouti  was  previously  known  to  be  a  sim- 
ple dominant  to  black,  i.e.  an  agouti  is  a  black  rabbit  plus 
an  additional  greying  factor  which  modifies  the  black  into 
agouti.  This  factor  we  will  denote  by  G,  and  we  will  use 
B 'for  the  black  factor.  Our  original  agouti  and  albino 
parents  we  may  therefore  regard  as  in  constitution 
GGCCBB  and  ggccBB  respectively.  Both  of  the  parents 
are  homozygous  for  black.  The  gametes  produced  by 
the  two  parents  are  GCB,  and  gcB,  and  the  constitution  of 
the  F!  animals  must  be  GgCcBB.  Being  heterozygous  for 
two  factors  they  will  produce  four  kinds  of  gametes  in 
equal  numbers,  viz.  GCB,  GcB,  gCB,  and  gcB.  The 
results  of  the  mating  of  two  such  similar  series  of  gametes 
when  the  FI  animals  are  bred  together  we  may  determine 
by  the  usual  " chessboard"  method  (Fig.  8).  Out  of  the 
1 6  squares  9  contain  both^and  G  in  addition  to  B.  Such 
animals  must  be  agoutis.  Three  squares  contain  C  but 
not  G.  Such  animals  must  be  coloured,  but  as  they  do 
not  contain  the  modifying  agouti  factor  their  colour  will 
be  black.  The  remaining  four  squares  do  not  contain  C, 
and  in  the  absence  of  this  colour-developing  factor  they 
must  all  be  albinos.  Theory  demands  that  the  three 
classes  agouti,  black,  and  albino  should  appear  in  F2  in  the 
ratio  9:3:4;  experiment  has  shown  that  these  are  the 
only  classes  that  appear,  and  that  the  proportions  in 
which  they  are  produced  accord  closely  with  the  theoret- 
ical expectation.  Put  briefly,  then,  the  explanation 


MENDELISM 


CHAP. 


of  this  case  is  that  all  the  animals  are  black,  and  that 
we  are  dealing  with  the  presence  and  absence  of  two 

factors,  a  colour  devel- 
oper (C),  and  a  colour 
modifier  (G),  both  act- 
as  it  were,  upon 
substratum  of  black. 
The  F2  generation 
really  consists  of  the 
four  classes  agoutis, 
blacks,  albino  agoutis, 
and  albino  blacks  in 
the  ratio  9:3:3:  i. 
But  since  in  the 
absence  of  the  colour 

Diagram  to  illustrate  the  nature  of  the  F2  generation 

which  may  arise  from  the  mating  of  agouti  with    developer  C  the  Colour 
albino  in  mice  or  rabbits. 

modifier  G  can  pro- 
duce no  visible  result,  the  last  two  classes  of  the  ratio  are 
indistinguishable,  and  our  F2  generation  conjes  to  consist 
of  three  classes  in  the  ratio  9:3:4,  instead  of  four  classes 
in  the  ratio  9:3:3:1. 

This  explanation  was  further  tested  by  experiments 
with  the  albinos.  In  an  F2  family  of  this  nature  there 
ought  to  be  three  kinds,  viz.  albinos  homozygous  for  G 
(GGccBB),  albinos  heterozygous  for  G  (GgccBB),  and 
albinos  \&J;hout  G  (ggccBB}.  These  albinos  are,  as  it 
were,  like  photographic  plates  exposed  but  undeveloped. 


v  INTERACTION   OF   FACTORS  53 

Their  potentialities  may  be  quite  different,  although  they 
all  look  alike,  but  this  can  only  be  tested  by  treating  them 
with  a  colour  developer.  In  the  case  of  the  mice  and  rab- 
bits the  potentiality  for  which  we  wish  to  test  is  the  pres- 
ence or  absence  of  the  factor  G,  and  in  order  to  develop 
the  colour  we  must  introduce  the  factor  C.  Our  de- 
veloper, therefore,  must  contain  C  but  not  G.  In  other 
words,  it  must  be  a  homozygous  black  mouse  or  rabbit, 
ggCCBB.  Since  such  an  animal  is  pure  for  C  it  must, 
when  mated  with  any  of  the  albinos,  produce  only  col- 
oured offspring.  And  since  it  does  not  contain  G  the  ap- 
pearance of  agoutis  among  its  offspring  must  be  attrib- 
uted to  the  presence  of  G  in  the  albino.  Tested  in  this 
way  the  F2  albinos  were  proved,  as  was  expected,  to  be  of 
three  kinds  :  (i)  those  which  gave  only  agouti,  i.e.  which 
were  homozygous  for  G;  (2)  those  which  gave  agoutis 
and  blacks  in  approximately  equal  numbers,  i.e.  which 
were  heterozygous  for  G ;  and  (3)  those  which  gave  only 
blacks,  and  therefore  did  not  contain  G. 

Though  albinos,  whether  mice,  rabbits,  rats,  or  other 
animals,  breed  true  to  albinism,  and  though  albinism  be- 
haves as  a  simple  recessive  to  colour,  yet  albinos  may  be 
of  many  different  sorts.  There  are  in  fact  just  as  many 
kinds  of  albinos  as  there  are  coloured  forms  —  neither 
more  nor  less.  And  all  these  different  kinds  of  albinos 
may  breed  together,  transmitting  the  various  colour  fac- 
tors according  to  the  Mendelian  scheme  of  inheritance, 


54  MENDELISM  CHAP. 

and  yet  the  visible  result  will  be  nothing  but  albinos. 
Under  the  mask  of  albinism  is  all  the  while  occurring  that 
segregation  of  the  different  colour  factors  which  would 
result  in  all  the  varieties  of  coloured  forms,  if  only  the 
essential  factor  for  colour  development  were  present. 
But  put  in  the  developer  by  crossing  with  a  pure  coloured 
form  and  their  variety  of  constitution  can  then  at  last 
become  manifest. 

So  far  we  have  dealt  with  cases  in  which  the  production 
of  a  character  is  dependent  upon  the  interaction  of  two 
factors.  But  it  may  be  that  some  characters  require  the 
simultaneous  presence  of  a  greater  number  of  factors  for 
their  manifestation,  and  the  experiments  of  Miss  Saunders 
have  shown  that  there  is  a  character  in  stocks  which  is  un- 
able to  appear  except  through  the  interaction  of  three 
distinct  factors.  Coloured  stocks  may  be  either  hoary, 
with  the  leaves  and  stem  covered  by  small  hairs,  or  they 
may  lack  the  hairy  covering,  in  which  case  they  are 
termed  glabrous.  Hoariness  is  dominant  to  glabrousness ; 
that  is  to  say,  there  is  a  definite  factor  which  can  turn  the 
glabrous  into  a  hoary  plant  when  it  is  present.  But  in 
families  where  coloured  and  white  stocks  occur  the  white 
are  always  glabrous,  while  the  coloured  plants  may  or  may 
not  be  hoary.  Now  colour  in  the  stock  as  in  the  sweet 
pea  has  been  proved  to  be  dependent  upon  the  interaction 
of  two  separate  factors.  Hence  hoariness  depends  upon 
three  separate  factors,  and  a  stock  cannot  be  hoary  unless 


INTERACTION  OF   FACTORS 


55 


it  contains  the  hoary  factor  in  addition  to  the  two  colour 
factors.  It  requires  the  presence  of  all  these  three  factors 
to  produce  the  hoary  character,  though  how  this  comes 
about  we  have  not  at  present  the  least  idea. 

A  somewhat  different  and  less  usual  form  of  inter- 
action between  factors  may  be  illustrated  by  a  case  in 
primulas  recently  worked  out  by  Bateson  and  Gregory. 
Like  the  common  primrose,  the  primula  exhibits  both 
pin-eyed  and  thrum-eyed  varieties.  In  the  former  the 
style  is  long,  and  the  centre  of  the  eye  is  formed  by  the  end 
of  the  stigma  which  more  or  less  plugs  up  the  opening  of 
the  corolla  (cf.  Fig.  9,  A) ;  in  the  latter  the  style  is  short 


FIG.  9. 

Sections  of  primula  flowers.  The  anthers  are  shown  as  black.  A,  "  pin  "  form  with 
long  style  and  anthers  set  low  down  ;  B, "  thrum  "  form  with  short  style  and  anthers 
set  higher  up ;  C,  homostyle  form  with  anthers  set  low  down  as  in  "  pin,"  but  with 
short  style.  This  form  only  occurs  with  the  large  eye. 

and  hidden  by  the  four  anthers  which  spring  from  higher 
up  in  the  corolla  and  form  the  centre  of  the  eye  (cf.  Fig. 
9,  B).  The  greater  part  of  the  "eye"  is  formed  by  the 
greenish-yellow  patches  on  each  petal  just  at  the  opening 


56  MENDELISM  CHAP. 

of  the  corolla.  In  most  primulas  the  eye  is  small,  but 
there  are  some  in  which  it  is  large  and  extends  as  a  flush 
over  a  considerable  part  of  the  petals  (Fig.  10).  Experi- 
ments showed  that  these  two  pairs  of  characters  behave 
in  simple  Mendelian  fashion,  short  style  (=  "thrum") 
being  dominant  to  long  style  (=  "pin")  and  small 
eye  dominant  to  large.  Besides  the  normal  long  and 
short  styled  forms,  there  occurs  a  third  form,  which  has 
been  termed  homostyle.  In  this  form  the  anthers  are 
placed  low  down  in  the  corolla  tube  as  they  are  in  the  long- 
styled  form,  but  the  style  remains  short  instead  of  reach- 
ing up  to  the  corolla  opening  (Fig.  9,  C).  In  the  course 


FIG.  10. 
Two  primula  flowers  showing  the  extent  of  the  small  and  of  the  large  eye. 

of  their  experiments  Bateson  and  Gregory  crossed  a  large- 
eyed  homostyle  plant  with  a  small-eyed  thrum  (=  short 
style).  The  Fl  plants  were  all  short  styled  with  small 


v  INTERACTION   OF   FACTORS  57 

eyes.  On  self- fertilisation  these  gave  an  F2  generation 
consisting  of  four  types,  viz.  short  styled  with  small  eyes, 
short  styled  with  large  eyes,  long  styled  with  small  eyes, 
and  homostyled  with  large  eyes.  The  notable  feature  of 
this  generation  is  the  appearance  of  long-styled  plants, 
which,  however,  occur  only  in  association  with  the  small 
eye.  The  proportions  in  which  these  four  types  appeared 
shows  that  the  presence  or  absence  of  but  two  factors 
is  concerned,  and  at  the  same  time  provides  the  key  to 
the  nature  of  the  homostyled  plants.  These  are  poten- 
tially long  styled,  and  the  position  of  the  anthers  is  that 
of  normal  long-styled  plants,  but  owing  to  some  interac- 
tion between  the  factors  the  style  itself  is  unable  to  reach 
its  full  development  unless  the  factor  for  the  small  eye  is 
present.  For  this  reason  long-styled  plants  with  the 

Short  style  \       f  Homo  style 
small  eye    j   §  \  large  eye 


Short  style 
small  eye 


Short  style  Short  style  Long  style  Homo  style 
small  eye     large  eye        ("pin")     large  eye 
.        (9)  (3)  (3)  (i) 

large  eye  are  always  of  the  homostyle  form.  What 
the  connecting-link  between  these  apparently  unrelated 
structures  may  be  we  cannot  yet  picture  to  ourselves,  any 
more  than  we  can  picture  the  relation  between  flower 


58  MENDELISM  CHAP,  v 

colour  and  hairiness  in  stocks.  It  is  evident,  however, 
that  the  conception  of  the  interaction  of  factors,  besides 
clearing  up  much  that  is  paradoxical  in  heredity,  prom- 
ises to  indicate  lines  of  research  which  may  lead  to  valu- 
able extensions  in  our  knowledge  of  the  way  in  which 
the  various  parts  of  the  living  organism  are  related  to  one 
another. 


CHAPTER  VI 

REVERSION 

As  soon  as  the  idea  was  grasped  that  characters  in 
plants  and  animals  might  be  due  to  the  interaction  of 
complementary  factors,  it  became  evident  that  this  threw 
clear  light  upon  the  hitherto  puzzling  phenomenon  of 
reversion.  We  have  already  seen  that  in  certain  cases 
the  cross  between  a  black  mouse  or  rabbit  and  an  albino, 
each  belonging  to  true  breeding  strains,  might  produce 
nothing  but  agoutis.  In  other  words,  the  cross  between 
the  black  and  the  white  in  certain  instances  results  in  a 
complete  reversion  to  the  wild  grey  form.  Expressed  in 
Mendelian  terms,  the  production  of  the  agouti  was  the 
necessary  consequence  of  the  meeting  of  the  factors  C  and 
G  in  the  same  zygote.  As  soon  as  they  are  brought  to- 
gether, no  matter  in  what  way,  the  reversion  is  bound  to 
occur,  ^ej/ersion,  therefore,  in  such  cases  we  may  regard 
as  the  bringing  together  of  complementary  factors  which 
had  somehow  in  the  course  of  evolution  become  separated 
from  one  another.  In  the  simplest  cases,  such  as  that  of 
the  black  and  the  white  rabbit,  only  two  factors  are  con- 
cerned, and  one  of  them  is  brought  in  from  each  of  the 

59 


60  MENDELISM  CHAP. 

two  parents.  But  in  other  cases  the  nature  of  the  rever- 
sion may  be  more  complicated  owing  to  a  larger  number 
of  factors  being  concerned,  though  the  general  princi- 
ple remains  the  same.  Careful  breeding  from  the  rever- 
sions will  enable  us  in  each  case  to  determine  the  number 
and  nature  of  the  factors  concerned,  and  in  illustration 
of  this  we  may  take  another  example  from  rabbits.  The 
Himalayan  rabbit  is  a  well-known  breed.  In  appearance 
it  is  a  white  rabbit  with  pink  eyes,  but  the  ears,  paws,  and 
nose  are  black  (PL  I.,  2).  The  Dutch  rabbit  is  another 
well-known  breed.  Generally  speaking,  the  anterior  por- 
tion of  the  body  is  white,  and  the  posterior  part  coloured. 
Anteriorly,  however,  the  eyes  are  surrounded  by  coloured 
patches  extending  up  to  the  ears,  which  are  entirely  col- 
oured. At  the  same  time  the  hind  paws  are  white  (cf. 
PL  I.,  i).  Dutch  rabbits  exist  in  many  varieties  of 
colour,  though  in  each  one  of  these  the  distribution  of 
colour  and  white  shows  the  same  relations.  In  the  ex- 
periments about  to  be  described  a  yellow  Diitch  rabbit 
was  crossed  with  a  Himalaya.  The  resuk  was  a  reversion 
to  the  wild  agouti  colour  (PL  I.,  3).  Some  of  the  F]  in- 
dividuals showed  white  patches,  while  others  were  self- 
coloured.  On  breeding  from  the  FI  animals  a  series  of 
coloured  forms  resulted  in  F2.  These  were  agoutis,  blacks, 
yellows,  and  sooty  yellows,  the  so-called  tortoise  shells  of 
the  fancy  (PL  L,  4-7).  In  addition  to  these  appeared 
Himalayans  with  either  black  points  or  with  lighter  brown- 


PLATE  I. 


vi  REVERSION  61 

ish  ones,  and  the  proportions  in  which  they  came  showed 
the  Himalayan  character  to  be  a  simple  recessive.  A  cer- 
tain number  of  the  coloured  forms  exhibited  the  Dutch 
marking  to  a  greater  or  less  extent,  but  as  its  inheritance 
in  this  set  of  experiments  is  complicated  and  has  not  yet 
been  worked  out,  we  may  for  the  present  neglect  it  and 
confine  our  attention  to  the  coloured  types  and  to  the 
Himalayans.  The  proportion  in  which  the  four  col- 
oured types  appeared  in  F2  was  very  nearly  9  agoutis,  3 
blacks,  3  yellows,  and  i  tortoiseshell.  Evidently  we  are 
here  dealing  with  two  factors:  (i)  the  grey  factor  (G), 
which  modifies  black  into  agouti,  or  tortoiseshell  into  yel- 
low; and  (2)  an  intensifying  factor  (7),  which  intensifies 
yellow  into  agouti  and  tortoiseshell  into  black.  It  may 

Yellow  X  Himalayan 


Agouti     X     Agouti 


Agouti    Yellow     Black    Tortoise  Himalayan 
(27)  (9)  (9)  S(sf  (16) 

be  mentioned  here  that  other  experiments  confirmed  the 
view  that  the  yellow  rabbit  is  a  dilute  agouti,  and  the 
tortoiseshell  a  dilute  black.  The  Himalayan  pattern  be- 
haves as  a  recessive  to  self-colour.  It  is  a  self-coloured 
black  rabbit  lacking  a  factor  that  allows  the  colour  to 
develop  except  in  the  points.  That  factor  we  may  denote 


62  MENDELISM  CHAP. 

by  X,  and  as  far  as  it  is  concerned  the  Himalayan  is  con- 
stitutionally xx.  The  Himalayan  contains  the  intensi- 
fying factor,  for  such  pigment  as  it  possesses  in  the  points 
is  full  coloured.  At  the  same  time  it  is  black,  i.e.  lacking 
in  the  factor  G.  With  regard  to  these  three  factors,  there- 
fore, the  constitution  of  the  Himalayan  is  ggllxx.  The 
last  character  which  we  have  to  consider  in  this  cross  is 
the  Dutch  character.  This  was  found  by  Hurst  to  be- 
have as  a  recessive  to  self-colour  (S),  and  for  our  present 
purpose  we  will  regard  it  as  differing  from  a  self-coloured 
rabbit  in  the  lack  of  this  factor.1  The  Himalayan  is  really 
a  self-coloured  animal,  which,  however,  is  unable  to  show 
itself  as  a  full  black  owing  to  its  not  possessing  the  fac- 
tor X.  The  results  of  breeding  experiments  then  sug- 
gest that  we  may  denote  the  Himalayan  by  the  formula 
ggllxxSS  and  the  yellow  Dutch  by  GGiiXXss.  Each 
lacks  two  of  the  factors,  upon  the  full  complement  of 
which  the  agouti  colour  depends.  By  crossing  them  the 
complete  series  GIXS  is  brought  into  the  same,zygote,  and 
the  result  is  a  reversion  to  the  colour  of  the  wild  rabbit. 
Most  of  the  instances  of  reversion  yet  worked  out  are 
those  in  which  colour  characters  are  concerned.  The 
sweet  pea,  however,  supplies  us  with  a  good  example  of 
reversion  in  structural  characters.  A  dwarf  variety 
known  as  the  "Cupid"  has  been  extensively  grown  for 

1  Hurst's  original  cross  was  between  a  Belgian  hare  and  an  albina 
Angora,  which  turned  to  out  be  a  masked  Dutch. 


vi  REVERSION  63 

some  years.  In  these  little  plants  the  internodes  are  very 
short  and  the  stems  are  few  in  number,  and  attain  to  a 
length  of  only  9-10  inches.  In  course  of  growth  they 
diverge  from  one  another,  and  come  to  lie  prostrate  on 
the  ground  (PI.  II.,  2).  Curiously  enough,  although  the 
whole  plant  is  dwarfed  in  other  respects,  this  does  not 
seem  to  affect  the  size  of  the  flower,  which  is  that  of  a 
normal  sweet  pea.  Another  though  less-known  variety  is 
the  "Bush"  sweet  pea.  Its  name  is  derived  from  its 
habit  of  growth.  The  numerous  stems  do  not  diverge 
from  one  another,  but  all  grow  up  side  by  side,  giving 
the  plant  the  appearance  of  a  compact  bush  (PI.  II.,  i). 
Under  ordinary  conditions  it  attains  a  height  of  3^-4 
feet.  A  number  of  crosses  were  made  between  the  Bush 

Bush  X  Cupid 

Tall F, 


Tall  Bush          Cupid         Cupid F2 

(procumbent)     (erect) 
(9)  (3)  (3)  (i) 

and  Cupid  varieties,  with  the  somewhat  unexpected  result 
that  in  every  instance  the  FI  plants  showed  complete 
reversion  to  the  size  and  habit  of  the  ordinary  tall  sweet 
pea  (PI.  II.,  3),  which  is  the  form  of  the  wild  plant  as  it 
occurs  in  Sicily  to-day.  The  F2  generation  from  these 
reversionary  tails  consisted  of  four  different  types,  viz. 


64  MENDELISM  CHAP. 

tails,  bushes,  Cupids  of  the  procumbent  type  like  the  orig- 
inal Cupid  parent,  and  Cupids  with  the  compact  upright 
Bush  habit  (PI.  II.,  4).  These  four  types  appeared  in  the 
ratio  9:3:3:1,  and  this,  of  course,  provided  the  clue  to 
the  nature  of  the  case.  The  characters  concerned  are  (i) 
long  internode  of  stem  between  the  leaves  which  is  domi- 
nant to  short  internode,  and  (2)  the  creeping  procum- 
bent habit  which  is  dominant  to  the  erect  bush-like  habit. 
Of  these  characters  length  of  internode  was  carried  by  the 
Bush,  and  the  procumbent  habit  by  the  original  Cupid 
parent.  The  bringing  of  them  together  by  the  cross 
resulted  in  a  procumbent  plant  with  long  internodes. 
This  is  the  ordinary  tall  sweet  pea  of  the  wild  Sicilian  type, 
reversion  here,  again,  being  due  to  the  bringing  together 
of  two  complementary  factors  which  had  somehow  be- 
come separated  in  the  course  of  evolution. 

To  this  interpretation  it  may  be  objected  that  the  or- 
dinary sweet  pea  is  a  plant  of  upright  habit.  This,  how- 
ever, is  not  true.  It  only  appears  so  because,the  conven- 
tional way  of  growing  it  is  to  train  it  up  sticks.  In  reality 
it  is  of  procumbent  habit,  with  divergent  stems  like  the 
ordinary  Cupid,  a  fact  which  can  easily  be  observed  by 
anyone  who  will  watch  them  grow  without  the  artificial 
aid  of  prepared  supports. 

The  cases  of  reversion  with  which  we  have  so  far  dealt 
have  been  cases  in  which  the  reversion  occurs  as  an  im- 
mediate result  of  a  cross,  i.e.  in  the  FI  generation.  This  is 


PLATE  II. 


i,  Bush  Sweet  Pea;  2,  Cupid  Sweet  Pea;  3,  Fj  reversionary  Tall; 

4,  Erect  Cupid  Sweet   Pea  ;  5,  Purple  Invincible ;  6,  Painted   Lady ; 

7,  Duke  of  Westminster  (hooded  standard). 


vi  REVERSION  65 

perhaps  the  commonest  mode  of  reversion,  but  instances 
are  known  in  which  the  reversion  that  occurs  when  two 
pure  types  are  crossed  does  not  appear  until  the  F2 
generation.  Such  a  case  we  have  already  met  with  in  the 
fowls'  combs.  It  will  be  remembered  that  the  cross  be- 
tween pure  pea  and  pure  rose  gave  walnut  combs  in  FI, 
while  in  the  F2  generation  a  definite  proportion,  i  in  16, 
of  single  combs  appeared  (cf.  p.  32).  Now  the  single 
comb  is  the  form  that  is  found  in  the  wild  jungle  fowl, 
which  is  generally  regarded  as  the  ancestor  of  the  domestic 
breeds.  If  this  is  so,  we  have  a  case  of  reversion  in  F2 ; 
and  this  in  the  absence  of  the  two  factors  brought  together 
by  the  rose-comb  and  pea-comb  parents.  Instead  of  the 
reversion  being  due  to  the  bringing  together  of  two  com- 
plementary factors,  we  must  regard  it  here  as  due  to  the 
association  of  two  complementary  absences.  To  this 
question,  however,  we  shall  revert  later  in  discussing  the 
origin  of  domesticated  varieties. 

There  is  one  other  instance  of  reversion  to  which  we 
must  allude.  This  is  Darwin's  famous  case  of  the  oc- 
casional appearance  of  pigeons  reverting  to  the  wild  blue 
rock  (Columba  lima),  when  certain  domesticated  races  are 
crossed  together.  As  is  well  known,  Darwin  made  use  of 
this  as  an  argument  for  regarding  all  the  domesticated  va- 
rieties as  having  arisen  from  the  same  wild  species.  The 
original  experiment  is  somewhat  complicated,  and  is 
shown  in  the  accompanying  scheme.  Essentially  it  lay  in 


66  MENDELISM  CHAP. 

following  the  results  flowing  from  crosses  between  blacks 
and  whites.  Experiments  recently  made  by  Staples- 
Browne  have  shown  that  this  case  of  reversion  also  can  be 
readily  interpreted  in  Mendelian  terms.  In  these  ex- 
Black  Barb  x  White  Fantail  Black  Barb  x  Spot l 

I  I 

Dark  X  Dark 

Among  the  offspring  one  very  similar 
to  the  wild  blue  rock. 

periments  the  cross  was  made  between  black  barbs  and 
white  fantails.  The  FI  birds  were  all  black  with  some 
white  splashes,  evidently  due  to  a  separate  factor  intro- 
duced by  the  fan  tail.  On  breeding  these  blacks  to- 
gether they  gave  an  F2  generation,  consisting  of  blacks 

Black  Y  White 
Barb     i    Fantail 


Black  Y  Black 


(White  Splashed) 


(White  Splashed) 


Black        Black          Blue          Blue         White 
White  Splashed)  (White  Splashed) 

~~$T  IsP  (4) 

(with  or  without  white  splashes) ,  blues  (with  or  without 
white  splashes),  and  whites  in  the  ratio  9:3:4.  The 
factors  concerned  are  colour  (C),  in  the  absence  of 

1  This  is  an  almost  white  bird,  the  colour  being  confined  to  the  tail 
and  the  characteristic  spot  on  the  head. 


VI 


REVERSION 


67 


which  a  bird  is  white,  and  a  black  modifier  (£),  in  the 
absence  of  which  a  coloured  bird  is  blue/  The  original 
black  barb  contained 
"both  of  these  factors, 
being  in  constitution 
CCBB.  The  fantail, 
however,  contained 
neither,  and  was  con- 
stitutionally ccbb. 
The  FI  birds  produced 
by  crossing  were  in 
constitution  CcBb, 
and  being  heterozy- 
gous for  two  factors 
produced  in  equal 
numbers  the  four 
sorts  of  gametes  CB, 

Cb,  cB,  cb.  The  results  of  two  such  series  of  gametes 
being  brought  together  are  shown  in  the  usual  way  in 
Fig.  ii.  A  blue  is  a  bird  containing  the  colour  factor 
but  lacking  the  black  modifier,  i.e.  of  the  constitution 
CCbb,  or  Ccbb,  and  such  birds  as  the  figure  shows  appear 
in  the  F2  generation  on  the  average  three  times  out  of 
sixteen.  Reversion  here  comes  about  in  F2,  when  the 
redistribution  of  the  factors  leads  to  the  formation  of 
zygotes  containing  one  of  the  two  factors  but  not  the 
other. 


FIG.  ii. 

Diagram  to  illustrate  the  appearance  of  the  rever- 
sionary blue  pigeon  in  F2  from  the  cross  of 
black  with  white. 


CHAPTER  VII 

DOMINANCE 

IN  the  cases  which  we  have  hitherto  considered  the 
presence  of  a  factor  produces  its  full  effect  whether  it  is 
introduced  by  both  of  the  gametes  which  go  to  form  the 
zygote,  or  by  one  of  them  alone.  The  heterozygous  tall 
pea  or  the  heterozygous  rose-combed  fowl  cannot  be  dis- 
tinguished from  the  homozygous  form  by  mere  inspection, 
however  close.  Breeding  tests  alone  can  decide  which 
is  the  heterozygous  and  which  the  homozygous  form. 
Though  this  is  true  for  the  majority  of  characters  yet 
investigated,  there  are  cases  known  in  which  the  hetero- 
zygous form  differs  in  appearance  from  either  parent. 
Among  plants  such  a  case  has  been  met  with  in  the  prim- 
ula. The  ordinary  Chinese  primula  (P .' sinensis)  (Fig. 
12)  has  large  rather  wavy  petals  much  crenated  at  the 
edges.  In  the  Star  Primula  (P.  stellata)  the  flowers  are 
much  smaller,  while  the  petals  are  flat  and  present  only  a 
terminal  notch  instead  of  the  numerous  crenations  of  P. 
sinensis.  The  heterozygote  produced  by  crossing  these 
forms  is  intermediate  in  size  and  appearance.  When  self- 
fertilised  such  plants  behave  in  simple  Mendelian  fashion, 

68 


CHAP.   VII 


DOMINANCE 


giving  a  generation  consisting  of  sinensis,  intermediates, 
and  stellata  in  the  ratio  1:2:1.  Subsequent  breeding 
from  these  plants  showed  that  both  the  sinensis  and  stel- 
lata which  appeared  in  the  F2  generation  bred  true,  while 


OOOO 


FIG.  12. 

Primula  flowers  to  illustrate  the  intermediate  nature  of  the  Ft  flower  when  sinensis  is 
crossed  with  stellata. 


the  intermediates  always  gave  all  three  forms  again  in  the 
same  proportion.  But  though  there  is  no  dominance  of 
the  character  of  either  parent  in  such  a  case  as  this,  the 
Mendelian  principle  of  segregation  could  hardly  have  a 
better  illustration. 


70  MENDELISM  CHAP. 

-   Among  birds  a  case  of  similar  nature  is  that  of  the  Blue 
Andalusian   fowl.     Fanciers   have   long   recognised   the 

Sinensis   X  Stellata 

I 

Intermediate F1 


Sinensis        Inter.  Inter.        Stellata F2 


Sinensis  sin.  int.  int.  StelL  Stellata F3 

Sinensis  Stellata- F4 

difficulty  of  getting  this  variety  to  breed  true.  Of  a  slaty 
blue  colour  itself  with  darker  hackles  and  with  black 
lacing  on  the  feathers  of  the  breast,  it  always  throws 

Blue  X  Blue 


Black 

Blue       X        Blue 

White 

I 

_ 

Black     Black      Blue       Blue     White     White 


Black X White 

Blue 
(all) 

"wasters"  of  two  kinds,  viz.  blacks,  and  whites  splashed 
with  black.  Careful  breeding  from  the  blues  shows  that 
the  three  sorts  are  always  produced  in  the  same  definite 


vii  DOMINANCE  71 

proportions,  viz.,  one  black,  two  blues,  one  splashed 
white.  This  at  once  suggests  that  the  black  and  the 
splashed  white  are  the  two  homozygous  forms,  and  that 
the  blues  are  heterozygous,  i.e.,  producing  equal  numbers 
of  "black"  and  "white  splashed"  gametes.  The  view 
was  tested  by  breeding  the  " wasters"  together  —  black 
with  black,  and  splashed  white  with  splashed  white  - 
and  it  was  found  that  each  bred  true  to  its  respective  type. 
But  when  the  black  and  the  splashed  white  were  crossed 
they  gave,  as  was  expected,  nothing  but  blues.  In  other 
words,  we  have  the  seeming  paradox  of  the  black  and  the 
splashed  white  producing  twice  as  many  blues  as  do  the 
blues  when  bred  together.  The  black  and  the  splashed 
white  " wasters"  are  in  reality  the  pure  breeds,  while  the 
"pure"  Blue  Andalusian  is  a  mongrel  which  no  amount 
of  selection  will  ever  be  able  to  fix. 

In  such  cases  as  this  it  is  obvious  that  we  cannot  speak 
of  dominance.  And  with  the  disappearance  of  this 
phenomenon  we  lose  one  criterion  for  determining  which 
of  the  two  .parent  forms  possesses  the  additional  factor. 
Are  we,  for  example,  to  regard  the  black  Andalusian  as  a 
splashed  white  to  which  has  been  added  a  double  dose  of 
a  colour-intensifying  factor,  or  are  we  to  consider  the 
white  splashed  bird  as  a  black  which  is  unable  to  show  its 
true  pigmentation  owing  to  the  possession  of  some  in- 
hibiting factor  which  prevents  the  manifestation  of  the 
black.  Either  interpretation  fits  the  facts  equally  well, 


72  MENDELISM  CHAP. 

and  until  further  experiments  have  been  devised  and  car- 
ried out  it  is  not  possible  to  decide  which  is  the  correct 
view. 

Besides  these  comparatively  rare  cases  where  the 
heterozygote  cannot  be  said  to  bear  a  closer  resemblance 
to  one  parent  more  than  to  the  other,  there  are  cases  in 
which  it  is  often  possible  to  draw  a  visible  distinction  be- 
tween the  heterozygote  and  the  pure  dominant.  There 
are  certain  white  breeds  of  poultry,  notably  the  White 
Leghorn,  in  which  the  white  behaves  as  a  dominant  to 
colour.  But  the  heterozygous  whites  made  by  crossing 
the  dominant  white  birds  with  a  pure  coloured  form  (such 
as  the  Brown  Leghorn)  almost  invariably  show  a  few  col- 
oured feathers  or  ''ticks"  in  their  plumage.  The  domi- 
nance of  white  is  not  quite  complete,  and  renders  it  pos- 
sible to  distinguish  the  pure  from  the  impure  dominant 
without  recourse  to  breeding  experiments. 

This  case  of  the  dominant  white  fowl  opens  up  another 
interesting  problem  in  connection  with  dominance.  By 
accepting  the  "  Presence  and  Absence  "  hypothesis  we  are 
committed  to  the  view  that  the  dominant  form  possesses 
an  extra  factor  as  compared  with  the  recessive.  The 
natural  way  of  looking  at  this  case  of  the  fowl  is  to  regard 
white  as  the  absence  of  colour.  But  were  this  so,  colour 
should  be  dominant  to  white,  which  is  not  the  case.  We 
are  therefore  forced  to  suppose  that  the  absence  of  colour 
in  this  instance  is  due  to  the  presence  of  a  factor  whose 


vii  DOMINANCE  73 

property  is  to  inhibit  the  production  of  colour  in  what 
would  otherwise  be  a  pure  coloured  bird.  On  this  view 
the  dominant  white  fowl  is  a  coloured  bird  plus  a  factor 
which  inhibits  the  development  of  the  colour.  The  view 
can  be  put  to  the  test  of  experiment.  We  have  already 
seen  that  there  are  other  white  fowls  in  which  white  is 
recessive  to  colour,  and  that  the  whiteness  of  such  birds 
is  due  to  the  fact  that  they  lack  a  factor  for  the  develop- 
ment of  colour.  If  we  denote  this  factor  by  C  and  our 
postulated  inhibitor  factor  in  the  dominant  white  bird  by 
/,  then  we  must  write  the  constitution  of  the  recessive 
white  as  ccii,  and  the  dominant  white  as  CCII.  We  may 
now  work  out  the  results  we  ought  to  obtain  when  a  cross 
is  made  between  these  two  pure  white  breeds.  The  con- 
stitution of  the  FI  bird  must  be  Ccii.  Such  birds  being 
heterozygous  for  the  inhibitor  factor,  should  be  whites 
showing  some  coloured  "  ticks.'7  Being  heterozygous  for 
both  of  the  two  factors  C  and  /,  they  will  produce  in  equal 
numbers  the  four  different  sorts  of  gametes  C/,  Ci,  ci,  ci. 
The  result  of  bringing  two  such  similar  series  of  gametes 
together  is  shown  in  Fig.  13.  Out  of  the  sixteen  squares, 
twelve  contain  / ;  these  will  be  white  birds  either  with  or 
without  a  few  coloured  ticks.  Three  contain  C  but  not  / : 
these  must  be  coloured  birds.  One  contains  neither  C 
nor  / ;  this  must  be  a  white.  From  such  a  mating 
we  ought,  therefore,  to  obtain  both  white  and  coloured 
birds  in  the  ratio  13  :  3.  The  results  thus  theoretically 


74 


MENDELISM 


CHAP. 


CI 
CI 


CI 
Ci 


CI 
cl 


CI 

ci 


Ci 
CI 


Ci 
ci 


ci 
CI 


ci 
Ci 


ci 
ci 


Cl 

CI 


Cl 

cl 


deduced  were  found  to  accord  with  the  actual  facts  of 
experiment.  The  FI  birds  were  all  "  ticked  "  whites,  and 
in  the  F2  generation  came  white  and  coloured  birds  in  the 

expected  ratio.    There 

seems,  therefore,  little 
reason  to  doubt  that 
the  dominant  white  is 
a  coloured  bird  in 
which  the  absence  of 
colour  is  due  to  the 
action  of  a  colour- 
inhibiting  factor, 
though  as  to  the 
nature  of  that  factor 
we  can  at  present 
make  no  surmise.  It 
is  probable  that  other 
facts,  which  at  first 

sight  do  not  appear  to  be  in  agreement  with  the  "  Presence 
and  Absence  "  hypothesis,  will  eventually  be  brought  into 
line  through  the  action  of  inhibitor  factors.  Such  a  case, 
for  instance,  is  that  of  bearded  and  beardless  wheats. 
Though  the  beard  is  obviously  the  additional  character, 
the  bearded  condition  is  recessive  to  the  beardless.  Prob- 
ably we  ought  to  regard  the  beardless  as  a  bearded  wheat 
in  which  there  is  an  inhibitor  that  stops  the  beard  from 
growing.  It  is  not  unlikely  that  as  time  goes  on  we  shall 


FIG.  13. 

Diagram  to  illustrate  the  nature  of  the  F2  generation 
from  the  cross  between  dominant  white  and 
recessive  white  fowls, 


VII 


DOMINANCE 


75 


find  many  more  such  cases  of  the  action  of  inhibitor 
factors,  and  we  must  be  prepared  to  find  that  the  same 
visible  effect  may  be  produced  either  by  the  addition  or 


FIG.  14. 

Ears  of  beardless  and  bearded  wheat,    The  beardless  condition  is  dominant 
to  the  bearded. 


by  the  omission  of  a  factor.  The  dominant  and  recessive 
white  poultry  are  indistinguishable  in  appearance.  Yet 
the  one  contains  a  factor  more  and  the  other  a  factor  less 
than  the  coloured  bird. 


76  MENDELISM  CHAP. 

A  phenomenon  sometimes  termed  irregularity  of  domi- 
nance has  been  investigated  in  a  few  cases.  In  certain 
breeds  of  poultry  such  as  Dorkings  there  occurs  an  extra 
toe  directed  backwards  like  the  hallux  (cf.  Fig.  15).  In 
some  families  this  character  behaves  as  an  ordinary 
dominant  to  the  normal,  giving  the  expected  3  :  i  ratio 
in  F2.  But  in  other  families  similarly  bred  the  pro- 
portions of  birds  with  and  without  the  extra  toe  appear 
to  be  unusual.  It  has  been  shown  that  in  such  a  family 
some  of  the  birds  without  the  extra  toe  may  nevertheless 
transmit  the  peculiarity  when  mated  with  birds  be- 
longing to  strains  in  which  the  extra  toe  never  occurs. 
Though  the  external  appearance  of  the  bird  generally 
affords  some  indication  of  the  nature  of  the  gametes 
which  it  is  carrying,  this  is  not  always  the  case. 
Nevertheless  we  have  reason  to  suppose  that  the  character 
segregates  in  the  gametes,  though  the  nature  of  these  can- 
not always  be  decided  from  the  appearance  of  the  bird 
which  bears  them. 

There  are  cases  in  which  an  apparent  .irregularity  of 
dominance  has  been  shown  to  depend  upon  another 
character,  as  in  the  experiments  with  sheep  carried  out  by 
Professor  Wood.  In  these  experiments  two  breeds  were 
crossed,  of  which  one,  the  Dorset,  is  horned  in  both  sexes, 
while  the  other,  the  Suffolk,  is  without  horns  in  either 
sex.  Whichever  way  the  cross  was  made  the  resulting 
FI  generation  was  similar;  the  rams  were  horned,  and 


vii  DOMINANCE  77 

the  ewes  were  hornless.  In  the  F2  generation  raised  from 
these  FI  animals  both  horned  and  hornless  types  ap- 
peared in  both  sexes  but  in  very  different  proportions. 


FIG.  15. 
Fowls'  feet.    On  the  right  a  normal  and  on  the  left  one  with  an  extra  toe. 

While  the  horned  rams  were  about  three  times  as  nu- 
merous as  the  hornless,  this  relation  was  reversed  among 
the  females,  in  which  the  horned  formed  only  about  one- 
quarter  of  the  total.  The  simplest  explanation  of  this 
interesting  case  is  to  suppose  that  the  dominance  of  the 
horned  character  depends  upon  the  sex  of  the  animal  — 
that  it  is  dominant  in  the  male  but  recessive  in  the 
female.  A  pretty  experiment  was  devised  for  putting 
this  view  to  the  test.  If  it  is  true,  equal  numbers  of 
gametes  with  and  without  the  horned  factor  must  be 
produced  by  the  F1  ewes,  while  the  factor  should  be  lack- 
ing in  all  the  gametes  of  the  hornless  F2  rams.  A  horn- 


78  MENDELISM  CHAP,  vn 

less  ram,  therefore,  put  to  a  flock  of  F!  ewes  should  give 
rise  to  equal  numbers  of  zygotes  which  are  heterozygous 
for  the  horned  character,  and  of  zygotes  in  which  it  is 

Dorset*  Suffolk       Suffolk   Dorset 

Ram         Ewe  Ram       Ewe 

I  X    Q  rfxf 


-F, 


?    9 


FIG.  16. 


Scheme  to  illustrate  the  inheritance  of  horns  in  sheep.      Heterozygous  males  shown 
dark  with  a  white  spot,  heterozygous  females  light  with  a  dark  spot  in  the  centre. 

completely  absent.  And  since  the  heterozygous  males 
are  horned,  while  the  heterozgyous  females  are  hornless, 
we  should  expect  from  this  mating  equal  numbers  of 
horned  and  hornless  rams,  but  only  hornless  ewes.  The 
result  of  the  experiment  confirmed  this  expectation.  Of 
the  ram  lambs  9  were  horned  and  8  were  hornless,  while 
all  the  1 1  ewe  lambs  were  completely  destitute  of  horns. 


- 


CHAPTER  VIII 

WILD  FORMS   AND  DOMESTIC  VARIETIES 

IN  discussing  the  phenomena  of  reversion  we  have 
seen  that  in  most  cases  such  reversion  occurs  when 
the  two  varieties  which  are  crossed  each  contain  certain 
factors  lacking  in  the  other,  of  which  the  full  complement 
is  necessary  for  the  production  of  the  reversionary  wild 
form.  This  at  once  suggests  the  idea  that  the  various 
domestic  forms  of  animals  and  plants  have  arisen  by  the 
omission  from  time  to  time  of  this  factor  or  of  that.  In 
some  cases  we  have  clear  evidence  that  this  is  the  most 
natural  interpretation  of  the  relation  between  the  culti- 
vated and  the  wild  forms.  Probably  the  species  in 
which  it  is  most  evident  is  the  sweet  pea  (Lathyrus  odora- 
tus).  We  have  already  seen  reason  to  suppose  that  as 
regards  certain  structural  features  the  Bush  variety  is 
a  wild  lacking  the  factor  for  the  procumbent  habit,  that 
the  Cupid  is  a  wild  without  the  factor  for  the  long  inter- 
node,  and  that  the  Bush  Cupid  is  a  wild  minus  both  these 
factors.  Nor  is  the  evidence  less  clear  for  the  many  colour 
varieties.  In  illustration  we  may  consider  in  more  de- 
tail a  case  in  which  the  cross  between  two  whites  resulted 

79 


8o  MENDELISM  CHAP. 

in  a  complete  reversion  to  the  purple  colour  characteris- 
tic of  the  wild  Sicilian  form  (PL  IV.)-  In  this  particular 
instance  subsequent  breeding  from  the  purples  resulted 
in  the  production  of  six  different  colour  forms  in  addi- 
tion to  whites.  The  proportion  of  the  coloured  forms  to 
the  whites  was  9 :  7  (cf.  p.  44),  but  it  is  with  the  relation 
of  the  six  coloured  forms  that  we  are  concerned  here. 
Of  these  six  forms  three  were  purples  and  three  were 
reds.  The  three  purple  forms  were  (i)  the  wild  bicolor 
purple  with  blue  wings  known  in  cultivation  as  the 
Purple  Invincible  (PL  IV.,  4);  (2)  a  deep  purple  with 
purple  wings  (PL  IV.,  5) ;  and  (3)  a  very  dilute  purple 
known  as  the  Pico  tee  (PL  IV.,  6).  Corresponding  to  these 
three  purple  forms  were  three  reds:  (i)  a  bicolor  red 
known  as  Painted  Lady  (PL  IV.,  7) ;  (2)  a  deep  red  with 
red  wings  known  as  Miss  Hunt  (PL  IV.,  8) ;  and  (3)  a  very 
pale  red  which  we  have  termed  Tinged  White 1  (PL  IV.,  9). 
In  the  F2  generation  the  total  number  of  purples  bore  to 
the  total  number  of  reds  the  ratio  3:1,  and  this  ratio  was 
maintained  for  each  of  the  corresponding  classes.  Purple, 
therefore,  is  dominant  to  red,  and  each  of  the  three  classes 
of  red  differs  from  its  corresponding  purple  in  not  possess- 
ing the  blue  factor  (B)  which  turns  it  into  purple.  Again, 

1  The  reader  who  searches  florists'  catalogues  for  these  varieties  will 
probably  experience  disappointment.  The  sweet  pea  has  been  much 
"improved"  in  the  past  few  years,  and  it  is  unlikely  that  the  modern 
seedsman  would  list  such  unfashionable  forms. 


PLATE  IV. 


i,  2,  Emily  Henderson;        3,  F!  reversionary  Purple;       4-10,  Various  F. 

forms  :     4,  Purple ;       5,  Deep  Purple ;       6,  Picotee ;       7,  Painted  Lady 

8,  Miss  Hunt ;    9,  Tinged  White ;    10,  White. 


vm          WILD  AND  DOMESTIC  VARIETIES  81 

the  proportion  in  which  the  three  classes  of  purples  ap- 
peared was  9  bicolors,  3  deep  purples,  4  picotees.  We 
are,  therefore,  concerned  here  with  the  operation  of  two 
factors :  (i)  a  light  wing  factor,  which  renders  the  bicolor 
dominant  to  the  dark  winged  form;  and  (2)  a  factor 
for  intense  colour,  which  occurs  in  the  bicolor  and  in 
the  deep  purple,  but  is  lacking  in  the  dilute  picotee. 
And  here  it  should  be  mentioned  that  these  conclusions 
rest  upon  an  exhaustive  set  of  experiments  involving  the 
breeding  of  many  thousands  of  plants.  In  this  cross, 
therefore,  we  are  concerned  with  the  presence  or  absence 
of  five  factors,  which  we  may  denote  as  follows :  — 

A  colour  base,  R. 

A  colour  developer,  C. 

A  purple  factor,  B. 

A  light  wing  factor,  L. 

A  factor  for  intense  colour,  7. 

On  this  notation  our  six  coloured  forms  are :  — 

(1)  Purple  bicolor      .         .         .  CRBLI.1 

(2)  Deep  purple         .         .         .  CRBII. 

(3)  Picotee         ....  CRBLi  or  CRBH. 

(4)  Red  bicolor  (=  Painted  Lady)  CRbLI. 

(5)  Deep  red  (  =  Miss  Hunt)     .  CRbll. 

(6)  Tinged  white        .         .         .  CRbLi  or  CRbli. 

It  will  be  noticed  in  this  series  that  the  various  coloured 

1  It  is  to  be  understood  that  wherever  a  given  factor  is  present  the 
plant  may  be  homozygous  or  heterozygous  for  it  without  alteration  in 
its  colour. 

G 


82  MENDELISM  CHAP. 

forms  can  be  expressed  by  the  omission  of  one  or  more 
factors  from  the  purple  bicolor  of  the  wild  type.  With 
the  complete  omission  of  each  factor  a  new  colour  type 
results,  and  it  is  difficult  to  resist  the  inference  that  the 
various  cultivated  forms  of  the  sweet  pea  have  arisen 
from  the  wild  by  some  process  of  this  kind.  Such  a  view 
tallies  with  what  we  know  of  the  behaviour  of  the  wild 
form  when  crossed  by  any  of  the  garden  varieties. 
Wherever  such  crossing  has  been  made  the  form  of  the 
hybrid  has  been  that  of  the  wild,  thus  supporting  the 
view  that  the  wild  contains  a  complete  set  of  all  the  dif- 
ferentiating factors  which  are  to  be  found  in  the  sweet 
pea. 

Moreover,  this  view  is  in  harmony  with  such  historical 
evidence  as  is  to  be  gleaned  from  botanical  literature, 
and  from  old  seedsmen's  catalogues.  The  wild  sweet  pea 
first  reached  England  in  1699,  having  been  sent  from 
Sicily  by  the  monk  Franciscus  Cupani  as  a  present  to  a 
certain  Dr.  Uvedale  in  the  county  of  Middlesex.  Some- 
what later  we  hear  of  two  new  varieties,  the  red  bicolor, 
or  Painted  Lady,  and  the  white,  each  of  which  may  be 
regarded  as  having  "sported"  from  the  wild  purple  by 
the  omission  of  the  purple  factor,  or  of  one  of  the  two 
colour  factors.  In  1793  we  find  a  seedsman  offering  also 
what  he  called  black  and  scarlet  varieties.  '  It  is  probable 
that  these  were  our  deep  purple  and  Miss  Hunt  varieties, 
and  that  somewhere  about  this  time  the  factor  for  the 


viii          WILD  AND  DOMESTIC  VARIETIES  83 

light  wing  (L)  was  dropped  out  in  certain  plants.  In 
1860  we  have  evidence  that  the  pale  purple  or  Pico  tee, 
and  with  it  doubtless  the  Tinged  White,  had  come  into 
existence.  This  time  it  was  the  factor  for  intense  colour 
which  had  dropped  out.  And  so  the  story  goes  on  until 
the  present  day,  and  it  is  now  possible  to  express  by  the 
same  simple  method  the  relation  of  the  modern  shades, 
of  purple  and  reds,  of  blues  and  pinks,  of  hooded  and 
wavy  standards,  to  one  another  and  to  the  original  wild 
form.  The  constitution  of  many  of  these  has  now  been 
worked  out,  and  to-day  it  would  be  a  simple  though  per- 
haps tedious  task  to  denote  all  the  different  varieties  by 
a  series  of  letters  indicating  the  factors  which  they  con- 
tain, instead  of  by  the  present  system  of  calling  them 
after  kings  and  queens,  and  famous  generals,  and  ladies 
more  or  less  well  known. 

From  what  we  know  of  the  history  of  the  various  strains 
of  sweet  peas  one  thing  stands  out  clearly.  The  new 
character  does  not  arise  from  a  pre-existing  variety  by 
any  process  of  gradual  selection,  conscious  or  otherwise. 
It  turns  up  suddenly  complete  in  itself,  •  and  thereafter 
it  can  be  associated  by  crossing  with  other  existing 
characters  to  produce  a  gamut  of  new  varieties.  If,  for 
example,  the  character  of  hooding  in  the  standard  (cf. 
PL  II.,  7)  suddenly  turned  up  in  such  a  family  as  that 
shown  on  Plate  IV.  we  should  be  able  to  get  a  hooded 
form  corresponding  to  each  of  the  forms  with  the  erect 


84  MENDELISM  CHAP. 

standard;  in  other  words,  the  arrival  of  the  new  form 
would  give  us  the  possibility  of  fourteen  varieties  instead 
of  seven.  As  we  know,  the  hooded  character  already 
exists.  It  is  recessive  to  the  erect  standard,  and  we  have 
reason  to  suppose  that  it  arose  as  a  sudden  sport  by  the 
omission  of  the  factor  in  whose  presence  the  standard 
assumes  the  erect  shape  characteristic  of  the  wild  flower. 
It  is  largely  by  keeping  his  eyes  open  and  seizing  upon 
such  sports  for  crossing  purposes  that  the  horticulturist 
" improves"  the  plants  with  which  he  deals.  How 
these  sports  or  mutations  come  about  we  can  now  sur- 
mise. They  must  owe  their  origin  to  a  disturbance  in 
the  processes  of  cell  division  through  which  the  gametes 
originate.  At  some  stage  or  other  the  normal  equal 
distribution  of  the  various  factors  is  upset,  and  some  of 
the  gametes  receive  a  factor  less  than  others.  From  the 
union  of  two  such  gametes,  provided  that  they  are  still 
capable  of  fertilisation,  comes  the  zygote  which  in  course 
of  growth  develops  the  new  character. 

Why  these  mutations  arise :  what  leads  to  the  sur- 
mised unequal  division  of  the  gametes:  of  this  we  know 
practically  nothing.  Nor  until  we  can  induce  the  pro- 
duction of  mutations  at  will  are  we  likely  to  understand 
the  conditions  which  govern  their  formation.  Never- 
theless there  are  already  hints  scattered  about  the  recent 
literature  of  experimental  biology  which  lead  us  to  hope 
that  we  may  know  more  of  these  matters  in  the  future. 


vm          WILD  AND  DOMESTIC  VARIETIES  85 

In  respect  of  the  evolution  of  its  now  multitudinous 
varieties,  the  story  of  the  sweet  pea  is  clear  and  straight- 
forward. These  have  all  arisen  from  the  wild  by  a  pro- 
cess of  continuous  loss.  Everything  was  there  in  the 
beginning,  and  as  the  wild  plant  parted  with  factor  after 
factor  there  came  into  being  the  long  series  of  derived 
forms.  Exquisite  as  are  the  results  of  civilization,  it  is 
by  the  degradation  of  the  wild  that  they  have  been 
brought  about.  How  far  are  we  justified  in  regarding 
this  as  a  picture  of  the  manner  in  which  evolution  works  ? 

There  are  certainly  other  species  in  which  we  must 
suppose  that  this  is  the  way  that  the  various  domesti- 
cated forms  have  arisen.  Such,  for  example,  is  the  case 
in  the  rabbit,  where  most  of  the  colour  varieties  are  re- 
cessive to  the  wild  agouti  form.  Such  also  is  the  case 
in  the  rat,  where  the  black  and  albino  varieties  and  the 
various  pattern  forms  are  also  recessive  to  the  wild  agouti 
type.  And  with  the  exception  of  a  certain  yellow  variety 
to  which  we  shall  refer  later,  such  is  also  the  case  with  the 
many  fancy  varieties  of  mice. 

Nevertheless  there  are  other  cases  in  which  we  must 
suppose  evolution  to  have  proceeded  by  the  interpola- 
tion of  characters.  In  discussing  reversion  on  crossing, 
we  have  already  seen  that  this  may  not  occur  until  the  F2 
generation,  as,  for  example,  in  the  instance  of  the  fowls' 
combs  (cf.  p.  65).  The  reversion  to  the  single  comb 
occurred  as  the  result  of  the  removal  of  the  two  factors 


86  MENDELISM  CHAP. 

for  rose  and  pea.  These  two  domesticated  varieties 
must  be  regarded  as  each  possessing  an  additional  factor 
in  comparison  with  the  wild  single-combed  bird.  Dur- 
ing the  evolution  of  the  fowl,  these  two  factors  must  be 
conceived  of  as  having  been  interpolated  in  some  way. 
And  the  same  holds  good  for  the  inhibitory  factor  on 
which,  as  we  have  seen,  the  dominant  white  character  of 
certain  poultry  depends.  In  pigeons,  too,  if  we  regard 
the  blue  rock  as  the  ancestor  of  the  domesticated  breeds, 
we  must  suppose  that  an  additional  melanic  factor  has 
arisen  at  some  stage.  For  we  have  already  seen  that 
black  is  dominant  to  blue,  and  the  characters  of  FI, 
together  with  the  greater  number  of  blacks  than  blues  in 
F2,  negatives  the  possibility  that  we  are  here  dealing  with 
an  inhibitory  factor.  The  hornless  or  polled  condition  of 
cattle,  again,  is  dominant  to  the  horned  condition,  and  if, 
as  seems  reasonable,  we  regard  the  original  ancestors 
of  domestic  cattle  as  having  been  horned,  we  have  here 
again  the  interpolation  of  an  inhibitory  factor-somewhere 
in  the  course  of  evolution. 

On  the  whole,  therefore,  we  must  be  prepared  to  admit 
that  the  evolution  of  domestic  varieties  may  come  about 
by  a  process  of  addition  of  factors  in  some  cases  and  of 
subtraction  in  others.  It  may  be  that  what  we  term 
additional  factors  fall  into  distinct  categories  from  the 
rest.  So  far,  experiment  seems  to  show  that  they  are 
either  of  the  nature  of  melanic  factors,  or  of  inhibitory 


vm          WILD  AND  DOMESTIC  VARIETIES  87 

factors,  or  of  reduplication  factors  as  in  the  case  of  the 
fowls'  combs.  But  while  the  data  remain  so  scanty, 
speculation  in  these  matters  is  too  hazardous  to  be 
profitable. 


CHAPTER  IX 

REPULSION  AND  COUPLING  OF  FACTORS 

ALTHOUGH  different  factors  may  act  together  to  pro- 
duce specific  results  in  the  zygote  through  their  inter- 
action, yet  in  all  the  cases  we  have  hitherto  considered 
the  heredity  of  each  of  the  different  factors  is  entirely 
independent.  The  interaction  of  the  factors  affects  the 
characters  of  the  zygote,  but  makes  no  difference  to  the 
distribution  of  the  separate  factors,  which  is  always  in 
strict  accordance  with  the  ordinary  Mendelian  scheme. 
Each  factor  in  this  respect  behaves  as  though  the  other 
were  not  present. 

A  few  cases  have  been  worked  out  in  which  the  dis- 
tribution of  the  different  factors  to  the  gametes  is  af- 
fected by  their  simultaneous  presence  in -.the  zygote. 
And  the  influence  which  they  are  able  to  exert  upon  one 
another  in  such  cases  is  of  two  kinds.  They  may  repel 
one  another,  refusing,  as  it  were,  to  enter  into  the  same 
zygote,  or  they  may  attract  one  another,  and,  becoming 
linked  together,  pass  into  the  same  gamete,  as  it  were 
by  preference.  For  the  moment  we  may  consider  these 

two  sets  of  phenomena  apart. 

88 


CHAP,  ix       REPULSION   AND    COUPLING  89 

One  of  the  best  illustrations  of  repulsion  between 
factors  occurs  in  the  sweet  pea.  We  have  already  seen 
that  the  loss  of  the  blue  or  purple  factor  (B)  from  the 
wild  bicolor  results  in  the  formation  of  the  red  bicolor 
known  as  Painted  Lady  (PL  IV.,  7).  Further,  we  have 
seen  that  the  hooded  standard  is  recessive  to  the  ordinary 
erect  standard.  The  omission  of  the  factor  for  the  erect 
standard  (E)  from  the  purple  bicolor  (PL  II.,  5)  results 
in  a  hooded  purple  known  as  Duke  of  Westminster 
(PL  II.,  7).  And  here  it  should  be  mentioned  that  in  the 
corresponding  hooded  forms  the  difference  in  colour  be- 
tween the  wings  and  standard  is  not  nearly  so  marked 
as  in  the  forms  with  the  erect  standard,  but  the  difference 
in  structure  appears  to  affect  the  colour,  which  becomes 
nearly  uniform.  This  may  be  readily  seen  by  comparing 
the  picture  of  the  purple  bicolor  on  Plate  II.  with  that 
of  the  Duke  of  Westminster  flower. 

Now  when  a  Duke  of  Westminster  is  mated  with  a 
Painted  Lady  the  factor  for  erect  standard  (E)  is  brought 
in  by  the  red,  and  that  for  blue  (B)  by  the  Duke,  and  the 
offspring  are  consequently  all  purple  bicolors.  Purples 
so  formed  are  all  heterozygous  for  these  two  factors, 
and  were  the  case  a  simple  one,  such  as  those  which  have 
already  been  discussed,  we  should  expect  the  F2  genera- 
tion to  consist  of  the  four  forms  :  erect  purple,  hooded 
purple,  erect  red,  and  hooded  red  in  the  ratio  9:3:  3:1. 
Such,  however,  is  not  the  case.  The  F2  generation 


go  MENDELISM  CHAP. 

actually  consists  of  only  three  forms,  viz.  erect  red, 
erect  purple,  and  hooded  purple,  and  the  ratio  in  which 
these  three  forms  occur  is  1:2:1.  No  hooded  red  has 
been  known  to  occur  in  such  a  family.  Moreover 
further  breeding  shows  that  while  the  erect  reds  and 
the  hooded  purples  always  breed  true,  the  erect  purples 

Painted  Lady  X  Duke  of  Westminster 


(erect  red) 


(hooded  purple) 


Purple  Invincible 
(srect  purple) 


Painted  Purple  Invincible         Duke  of 

Lady  Westminster 

(I)  (2)  (I) 

in  such  families  never  breed  true,  but  always  behave 
like  the  original  FI  plant,  giving  the  three  forms  again 
in  the  ratio  1:2:1.  Yet  we  know  that  there  is  no  diffi- 
culty in  getting  purple  bicolors  to  breed  true. from  other 
families ;  and  we  know  also  that  hooded  red, sweet  peas 
exist  in  other  strains. 

On  the  assumption  that  there  exists  a  repulsion  be- 
tween the  factors  for  erect  standard  and  blue  in  a  plant 
which  is  heterozygous  for  both,  this  peculiar  case  receives 
a  simple  explanation.  The  constitutions  of  the  erect 
red  and  the  hooded  purple  are  EEbb  and  eeBB  respectively 
and  that  of  the  FI  erect  purple  is  EeBb.  Now  let  us 
suppose  that  in  such  a  zygote  there  exists  a  repulsion 


IX 


REPULSION   AND    COUPLING 


between  E  and  B,  such  that  when  the  plant  forms  gametes 
these  two  factors  will  not  go  into  the  same  gamete. 
On  this  view  it  can  only  form  two  kinds  of  gametes,  viz. 
Eb  and  eB,  and  these,  of  course,  will  be  formed  in  equal 
numbers.  Such  a  plant  on  self-fertilisation  must  give 

EEbb  eeBB  Parents 


9  gametes  of  1 

Eb 
Eb 
eB 
eB 

»-» 

EEbb 
EeBb 
EeBb 
eeBB 

111     v 

^  

Eb" 
eB 
Eb 
eB  . 

generation 

the  zygotic  series  EEbb  +  2  EeBb  +  eeBB,  i.e.  i  erect  red, 
2  erect  purples,  and  i  hooded  purple.  And  because  the 
erect  reds  and  the  hooded  purples  are  respectively  homo- 
zygous  for  E  and  B,  they  must  thenceforward  breed  true. 
The  erect  purples,  on  the  other  hand,  being  always  formed 
by  the  union  of  a  gamete  Eb  with  a  gamete  eB,  are 
always  heterozygous  for  both  of  these  factors.  They 
can,  consequently,  never  breed  true,  but  must  always 
give  erect  reds,  erect  purples,  and  hooded  purples  in  the 
ratio  1:2:1.  The  experimental  facts  are  readily  ex- 
plained on  the  assumption  of  repulsion  between  the  two 


92  MENDELISM  CHAP. 

factors  B  and  E  during  the  formation  of  the  gametes  in 
a  plant  which  is  heterozygous  for  both. 

Other  similar  cases  of  factorial  repulsion  have  been 
demonstrated  in  the  sweet  pea,  and  two  of  these  are  also 
concerned  with  the  two  factors  with  which  we  have  just 
been  dealing.  Two  distinct  varieties  of  pollen  grains 
occur  in  this  species,  viz.  the  ordinary  oblong  form  and 
a  rather  smaller  rounded  grain.  The  former  is  dominant 
to  the  latter.1  When  a  cross  is  made  between  a  purple 
with  round  pollen  and  a  red  with  long  pollen  the  FI  plant 
is  a  long  pollened  purple.  But  the  F2  generation  con- 
sists of  purples  with  round  pollen,  purples  with  long 
pollen,  and  reds  with  long  pollen  in  the  ratio  1:2:1. 
No  red  with  round  pollen  appears  in  F2  owing  to  repul- 
sion between  the  factors  for  purple  (B)  and  for  long  pollen 
(L).  Similarly  plants  produced  by  crossing  a  red 
hooded  long  with  a  red  round  having  an  erect  standard 
give  in  FI  long  pollened  reds  with  an  erect  standard, 
and  these  in  F2  produce  the  three  types,  round  pollened 
erect,  long  pollened  erect,  and  long  pollened  hooded,  in 
the  ratio  1:2:1.  The  repulsion  here  is  between  the  long 
pollen  factor  (L)  and  the  factor  for  the  erect  standard 


1  It  should  be  mentioned  that  as  the  shape  of  the  pollen  coat,  like 
that  of  the  seed  coat,  is  a  maternal  character,  all  the  grains  of  any  given 
plant  are  either  long  or  else  round.  The  two  kinds  do  not  occur  together 
on  the  same  plant. 


ix  REPULSION  AND   COUPLING  93 

Yet  another  similar  case  is  known  in  which  we  are  con- 
cerned with  quite  different  factors.  In  some  sweet  peas 
the  axils  whence  the  leaves  and  flower  stalks  spring  from 
the  main  stem  are  of  a  deep  red  colour.  In  others  they 
are  green.  The  dark  pigmented  axil  is  dominant  to  the 
light  one.  Again,  in  some  sweet  peas  the  anthers  are 
sterile,  setting  no  pollen,  and  this  condition  is  recessive 
to  the  ordinary  fertile  condition.  When  a  sterile  plant 
with  a  dark  axil  is  crossed  by  a  fertile  plant  with  a  light 
axil,  the  FI  plants  are  all  fertile  with  dark  axils.  But 
such  plants  in  F2  give  fer tiles  with  light  axils,  fer tiles  with 
dark  axils,  and  steriles  with  dark  axils  in  the  ratio  1:2:1. 
No  light  axilled  steriles  appear  from  such  a  cross  owing 
to  the  repulsion  between  the  factor  for  dark  axil  (D)  and 
that  for  the  fertile  anther  (F). 

These  four  cases  have  already  been  found  in  the  sweet 
pea,  and  similar  phenomena  have  been  met  with  by 
Gregory  in  primulas.  To  certain  seemingly  analogous 
cases  in  animals  where  sex  is  concerned  we  shall  refer 
later. 

Now  all  of  these  four  cases  present  a  common  feature 
which  probably  has  not  escaped  the  attention  of  the 
reader.  In  all  of  them  the  original  cross  was  such  as  to 
introduce  one  of  the  repelling  factors  with  each  of  the  two 
parents.  If  we  denote  our  two  factors  by  A  and  B,  the 
crosses  have  always  been  of  the  nature  AAbbxaaBB. 
Let  us  now  consider  what  happens  when  both  of  the 


94  MENDELISM  CHAP. 

factors,  which  in  these  cases  repel  one  another,  are  in- 
troduced by  one  of  the  parents,  and  neither  by  the  other 
parent.  And  in  particular  we  will  take  the  case  in  which 
we  are  concerned  with  purple  and  red  flower  colour,  and 
with  long  and  round  pollen,  i.e.  with  the  factors  B  and  L. 
When  a  purple  long  (BBLL)  is  crossed  with  a  red  round 
(bbll)  the  FI  (BbLl)  is  a  purple  with  long  pollen,  identical 
in  appearance  with  that  produced  by  crossing  the  long 
pollened  red  with  the  round  pollened  purple.  But  the 
nature  of  the  F2  generation  is  in  some  respects  very  dif- 
ferent. The  ratio  of  purples  to  reds  and  of  longs  to 
rounds  is  in  each  case  3  :  i,  as  before.  But  instead  of  an 
association  between  the  red  and  the  long  pollen  characters 
the  reverse  is  the  case.  The  long  pollen  character  is 
now  associated  with  purple  and  the  round  pollen  with  red. 
The  association,  however,  is  not  quite  complete,  and  the 
examination  of  a  large  quantity  of  similarly  bred  material 
shows  that  the  purple  longs  are  about  twelve  times  as 
numerous  as  the  purple  rounds,  while  the  red  rounds 
are  rather  more  than  three  times  as  many  as  the  red  longs. 
Now  this  peculiar  result  could  be  brought  about  if  the 
gametic  series  produced  by  the  FI  plant  consisted  of 
jBL+iBl+ibL  +  jbl  out  of  every  16  gametes.  Fer- 
tilization between  two  such  similar  series  of  16  gametes 
would  result  in  256  plants,  of  which  177  would  be  purple 
longs,  15  purple  rounds,  15  red  longs,  and  49  red  rounds 
—  a  proportion  of  the  four  different  kinds  very  close  to 


ix  REPULSION  AN'D   COUPLING 


95 


that  actually  found  by  experiment.  It  will  be  noticed 
that  in  the  whole  family  the  purples  are  to  the  reds  as 
3:1,  and  the  longs  are  also  three  times  as  numerous  as 
the  rounds.  The  peculiarity  of  the  case  lies  in  the  dis- 
tribution of  these  two  characters  with  regard  to  one  an- 
other. In  some  way  or  other  the  factors  for  blue  and 
for  long  pollen  become  linked  together  in  the  cell  divisions 
that  give  rise  to  the  gametes,  but  the  linking  is  not  com- 
plete. This  holds  good  for  all  the  four  cases  in  which 
repulsion  between  the  factors  occurs  when  one  of  the 
two  factors  is  introduced  by  each  of  the  parents.  When 
both  of  the  factors  are  brought  into  the  cross  by  the  same 
parent  we  get  coupling  between  them  instead  of  repulsion. 
The  phenomena  of  repulsion  and  coupling  between 
separate  factors  are  intimately  related,  though  hitherto 
we  have  not  been  able  to  suggest  why  this  should  be  so. 
Nor  for  the  present  can  we  suggest  why  certain  factors 
should  be  linked  together  in  the  peculiar  way  that  we 
have  reason  to  suppose  that  they  are  during  the  process 
of  the  formation  of  the  gametes.  Nevertheless  the 
phenomena  are  very  definite,  and  it  is  not  unlikely  that 
a  further  study  of  them  may  throw  important  light  on 
the  architecture  of  the  living  cell. 

APPENDIX   TO   CHAPTER  IX 

As  it  is  possible  that  some  readers  may  care,  in  spite  of  its  com- 
plexity, to  enter  rather  more  fully  into  the  peculiar  phenomenon 


96  MENDELISM  CHAP. 

of  the  coupling  of  characters,  I  have  brought  together  some  further 
data  in  this  Appendix.  In  the  case  we  have  already  considered, 
where  the  factors  for  blue  colour  and  long  pollen  are  concerned,  we 
have  been  led  to  suppose  that  the  gametes  produced  by  the  hetero- 
zygous plant  are  of  the  nature  7  BL :  i  Bl:  i  bL :  7  bl.  Such  a  series 
of  ovules  fertilised  by  a  similar  series  of  pollen  grains  will  give 
a  generation  of  the  following  composition :  — 

49  BBLL  +  7  BBLl  +  7  BbLL  +  49  BID, 

+  7  BBLl  +  7  BbLL  +      BbLl 

+       BbLl 

-f  49  BbLl 

177  purple,  long 

+  BBll  +  7  BUI  4-  bbLL  +  7  bbU  +  49  bbtt 
+  7  BUI      +  7  bbLl 

15  purple,  15  red, 

round  long 

and  as  this  theoretical  result  fits  closely  with  the  actual  figures 
obtained  by  experiment  we  have  reason  for  supposing  that  the 
heterozygous  plant  produces  a  series  of  gametes  in  which  the  factors 
are  coupled  in  this  way.  The  intensity  of  the  coupling,  however, 
varies  in  different  cases.  Where  we  are  dealing  with  another,  viz. 
fertility  (F)  and  the  dark  axil  (/?),  the  experimental  numbers  accord 
with  the  view  that  the  gametic  series  is  here  1 5  FD :  i  Fd :  i  jD : 
15  fd.  The  coupling  is  in  this  instance  more  intense.  In  the  case 
of  the  erect  standard  (E)  and  blueness  (B)  the  coupling  is  even  more 
intense,  and  the  experimental  evidence  available  at  present  points 
to  the  gametic  series  here  being  63  Eb :  i  EB  :  i  eB:6$  eb.  There 
is  evidence  also  for  supposing  that  the  intensity  of  the  coupling 
may  vary  in  different  families  for  the  same  pair  of  factors.  The 
coupling  between  blue  and  long  pollen  is  generally  on  the  7  :  i :  i :  7 


IX 


REPULSION  AND   COUPLING 


97 


basis,  but  in  some  cases  it  may  be  on  the  15  :  i  :  i  :  15  basis.  But 
though  the  intensity  of  the  coupling  may  vary  it  varies  in  an  orderly 
way.  If  A  and  B  are  the  two  factors  concerned,  the  results  ob- 
tained in  F2  are  explicable  on  the  assumption  that  the  ratio  of  the 
•four  sorts  of  gametes  produced  is  a  term  of  the  series  — 


sab 
7  ab 
$  ab,  etc.,  etc. 


In  such  a  series  the  number  of  gametes  containing  A  is  equal  to  the 
number  lacking  A  ,  and  the  same  is  true  for  B.  Consequently  the 
number  of  zygotes  formed  containing  A  is  three  times  as  great  as 
the  number  of  zygotes  which  do  not  contain  A  ;  and  similarly  for 
B.  The  proportion  of  dominants  to  recessives  in  each  case  is  3  :  i. 
It  is  only  in  the  distribution  of  the  characters  with  relation  to 
one  another  that  these  cases  differ  from  a  simple  Mendelian  case. 
As  the  study  of  these  series  presents  another  feature  of  some 
interest,  we  may  consider  it  in  a  little  more  detail.  In  the  accom- 
panying table  are  set  out  the  results  produced  by  these  different 
series  of  gametes.  The  series  marked  by  an  asterisk  have  already 
been  demonstrated  experimentally.  The  first  term  in  the  series, 


13  iS 

Distribution  of 

<o*1 

oG  § 

Factors  in  Gametic 

6  S.§ 

Form  of  F2  Generation. 

z  3 

^O.S 

Series 

*N2 

4 

AB.  Ab.  aB.  ab. 
i  :      :      :     i 

16 

AB.          Ab.            aB.           ab. 
9331 

8 

3  :      :      :     3 

64 

49              7              7                9 

16 

7  :      :      :     7 

256 

177             IS            15              49* 

32 

IS  :      :      :  15 

1024 

737            3i            3i            225  * 

64 

3i  :      :      :  3i 

4096 

3009            63            63            961 

128 

63  :      :      :  63 

16384 

12161          127          127          3969* 

2tt 

(•»  —  i)  :  i  :      :  (»  —  i) 

4«2 

3«2  —  (2M  —  i)    (2M  —  i)    (291  —  i)    M2-(2W  —  I 

98  MENDELISM  CHAP,  ix 

in  which  all  the  four  kinds  of  gametes  are  produced  in  equal  num- 
bers is,  of  course,  that  of  a  simple  Mendelian  case  where  no  coupling 
occurs. 

Now,  as  the  table  shows,  it  is  possible  to  express  the  gametic  series 
by  a  general  formula  (n  +  i)  AB  +  Ab-\-  aB  +  (n  —  i)ab,  where  2  n 
is  the  total  number  of  the  gametes  in  the  series.  A  plant  produc- 
ing such  a  series  of  gametes  gives  rise  to  a  family  of  zygotes  in 
which  3  n2  —  (2  n  —  i)  show  both  of  the  dominant  characters  and  w2 
—  (2  w  —  i)  show  both  of  the  recessive  characters,  while  the  number 
of  the  two  classes  which  each  show  one  of  the  two  dominants  is 
(2  n  —  i).  When  in  such  a  series  the  coupling  becomes  closer  the 
value  of  n  increases,  but  in  comparison  with  w2  its  value  becomes 
less  and  less.  The  larger  n  becomes  the  more  negligible  is  its  value 
relati  vely  to  w2.  If,  therefore,  the  coupling  were  very  close,  the  series 
3  w2  —  (2  n—  i)  :  (2  n  —  i) :  (zn  —  i)  :  nz  —  (2  n  —  i)  would  approx- 
imate more  and  more  to  the  series  3  w2 :  w2,  i.e.  to  a  simple  3  :  i 
ratio.  Though  the  point  is  probably  of  more  theoretical  than 
practical  interest,  it  is  not  impossible  that  some  of  the  cases  which 
have  hitherto  been  regarded  as  following  a  simple  311  ratio  will 
turn  out  on  further  analysis  to  belong  to  this  more  complicated 
scheme. 


CHAPTER  X 

SEX 

IN  their  simplest  expression  the  phenomena  exhibited 
by  Mendelian  characters  are  sharp  and  clean  cut.  Clean 
cut  and  sharp  also  are  the  phenomena  of  sex.  It  was 
natural,  therefore,  that  a  comparison  should  have  been 
early  instituted  between  these  two  sets  of  phenomena. 
As  a  general  rule,  the  cross  between  a  male  and  a  female 
results  in  the  production  of  the  two  sexes  in  approxi- 
mately equal  numbers.  The  cross  between  a  hetero- 
zygous dominant  and  a  recessive  also  leads  to  equal 
numbers  of  recessives  and  of  heterozygous  dominants. 
Is  it  not,  therefore,  possible  that  one  of  the  sexes  is  hetero- 
zygous for  a  factor  which  is  lacking  in  the  other,  and  that 
the  presence  or  absence  of  this  factor  determines  the  sex 
of  the  zygote  ?  The  results  of  some  recent  experiments 
would  appear  to  justify  this  interpretation,  at  any  rate  in 
particular  cases.  Of  these,  the  simplest  is  that  of  the 
common  currant  moth  (Abraxas  grossulariata) ,  of  which 
there  exists  a  pale  variety  (Fig.  17)  known  as  lacticolor. 
The  experiments  of  Doncaster  and  Raynor  showed  that 
the  variety  behaved  as  a  simple  recessive  to  the  normal 
form.  But  the  distribution  of  the  dominants  and  reces- 

99 


100 


MENDELISM 


CHAP. 


sives  with  regard  to  the  sexes  was  peculiar.  The  original 
cross  was  between  a  lacticolor  female  and  a  normal  male. 
All  the  F!  moths  of  both  sexes  were  of  the  normal  grossu- 
lariata  type.  The  FI  insects  were  then  paired  together 


FIG.  17. 

Abraxas  grossulariata,  the  common  currant  moth,  and  (on  the  right)  its  paler  lacticolor 

variety. 

and  gave  a  generation  consisting  of  3  normals :  i  lacti- 
color. But  all  the  lacticolor  were  females,  and  all  the 
males  were  of  the  normal  pattern.  It  was,  however, 
found  possible  to  obtain  the  lacticolor  male  by  mating  a 


Lacticclor 

9 


Grossulariata 
(J 


Lact.  <?      X      Gr.9       X        Gr.C?     X  Lact.  9 


Gr.C?  Lact?  Gr.C?  Gr.C?  Gr.9  Lact.9  Gr.C?  Lact.C?  Gr.9  Lact.9 

lacticolor  female  with  the  FI  male.  The  family  resulting 
from  this  cross  consisted  of  normal  males  and  normal 
females,  lacticolor  males  and  lacticolor  females,  and  the 


IOI 


four  sorts  were  produced  in  approximately  equal  numbers. 
In  such  a  family  there  was  no  special  association  of  either 
of  the  two  colour  varieties  with  one  sex  rather  than  the 
other.  But  the  reverse  cross,  FI  female  by  lacticolor 
male,  gave  a  very  different  result.  As  in  the  previous 
cross,  such  families  contained  equal  numbers  of  the 
normal  form  and  of  the  recessive  variety.  But  all  of  the 
normal  grossulariata  were  males,  while  all  the  laclicolor 
were  females.  Now  this  seemingly  complex  collection 
of  facts  is  readily  explained  if  we  make  the  following 
three  assumptions :  — 

(1)  The  grossulariata  character  (G)  is  dominant  to  the 
lacticolor  character  (g) .     This  is  obviously  justified  tiy  the 
experiments,  for,  leaving  the  sex  distribution  out  of  ac- 
count, we  get  the  expected  3  :  i  ratio  from  FI  X  FI,  and 
also  the  expected  ratio  of  equality  when  the  heterozygote 
is  crossed  with  the  recessive. 

(2)  The  female  is  heterozygous  for  a  dominant  factor 
(F)  which  is  lacking  in  the  male.     The  constitution  of  a 
female  is  consequently  Ff,  and  of  a  male  ff.     This  as- 
sumption is  in  harmony  with  the  fact  that  the  sexes  are 
produced  in  approximately  equal  numbers. 

(3)  There  exists  repulsion  between  the  factors  G  and  F 
in  a  zygote  which  is  heterozygous  for  them  both.     Such 
zygotes   (FfGg)   must  always  be  females,  and  on  this 
assumption  will  produce  gametes  Fg  and  fG  in  equal 
numbers. 


102  MENDELISM  CHAP. 

We  may  now  construct  a  scheme  for  comparison  with 
that  on  page  100  to  show  how  these  assumptions  explain 
the  experimental  results.  The  original  parents  were  lacti- 
color  female  and  grossulariata  male,  which  on  our  assump- 
tions must  be  Ffgg  and  ffGG  respectively  in  constitution. 

Ffgg[9] 


fg. 


FfGg[f] 

Fg 

fs 


Ffgg  FfGg  ffGg  ffGG   FfGg  Ffgg  ffGg  ffgg 
[9]       W]        [91     [»]    «]   W]     Ef]    [9] 


FIG.  i  8. 

Scheme  of  inheritance  in  the  Yl  and  F2  generations  resulting  from  the  cross,  of  lacticolor 
female  with  grossulariata  male.  The  character  of  each  individual  is  represented  by  the 
sex  signs  in  brackets,  the  black  being  grossulariat  in  appearance  and  the  light  ones  lac- 
ticolor. 

Since  the  female  is  always  heterozygous  for  F,  her  gametes 
must  be  of  two  kinds,  viz.  Fg  and  fg,  while  those  of  the 
pure  grossulariata  male  must  be  all  fG.  When  an  ovum 
Fg  is  fertilised  by  a  spermatozoon  fG,  the  resulting  zygote, 
FfGg,  is  heterozygous  for  both  F  and  G,  and  in  appearance 
is  a  female  grossulariata.  The  zygote  resulting  from  the 
fertilisation  of  an  ovum/g  by  a  spermatozoon  fG  is  hetero- 
zygous for  G,  but  does  not  contain  F,  and  therefore  is  a 
male  grossulariata.  Such  a  male  being  in  constitution 


SEX 


103 


ffGg  must  produce  gametes  of  two  kinds,  fG  and  fg,  in 
equal  numbers.  And  since  we  are  assuming  repulsion 
between  F  and  G,  the  FI  female  being  in  constitution 
FfGg,  must  produce  equal  numbers  of  gametes  Fg  and 
fG.  For  on  our  assumption  F  and  G  cannot  enter  into 
the  same  gamete.  The  series  of  gametes  produced  by 
the  FI  moths,  therefore,  are/G,/g  by  the  male  and  Fg,fG 
by  the  female.  The  resulting  F2  generation  consequently 
consists  of  the  four  classes  of  zygotes  Ffgg,  FfGg,  ffGg,  and 
ffGG  in  equal  numbers.  In  other  words,  the  sexes  are 
produced  in  equal  numbers,  the  proportion  of  normal 
grossulariata  to  lacticolor  is  3  :  i,  and  all  of  the  lacticolor 
are  females ;  that  is  to  say,  the  results  worked  out  on  our 
assumptions  accord  with  those  actually  produced  by  ex- 
periment. We  may  now  turn  to  the  results  which  should 
be  obtained  by  crossing  the  FI  moths  with  the  lacticolor 
variety.  And  first  we  will  take  the  cross  lacticolor  female 
X  FI  male.  The  gametes  produced  by  the  lacticolor 
female  we  have  already  seen  to  be  Fg  and  fg,  while  those 
produced  by  the  FI  male  are  fG  and  fg.  The  bringing 
together  of  these  two  series  of  gametes  must  result  in 
equal  numbers  of  the  four  kinds  of  zygotes  FfGg,  Ffgg, 
ffGg,  and  ffgg,  i.e.  of  female  grossulariata  and  lacticolor, 
and  of  male  grossulariata  and  lacticolor  in  equal  numbers. 
Here,  again,  the  calculated  results  accord  with  those  of 
experiment.  Lastly,  we  may  examine  what  should  hap- 
pen when  the  FI  female  is  crossed  with  the  lacticolor 


104  MENDELISM  CHAP. 

male.  The  FI  female,  owing  to  the  repulsion  between 
F  and  G,  produces  only  the  two  kinds  of  ova  Fg  and 
/G,  and  produces  them  in  equal  numbers.  Since 
the  lacticolor  male  can  contain  neither  F  nor  G,  all 
of  its  spermatozoa  must  be  fg.  The  results  of  such 
a  cross,  therefore,  should  be  to  produce  equal  num- 
bers of  the  two  kinds  of  zygote  Ffgg  and  ffGg,  i.e. 
of  lacticolor  females  and  of  grossulariata  males.  And 
this,  as  we  have  already  seen,  is  the  actual  result 
of  such  a  cross. 

Before  leaving  the  currant  moth  we  may  allude  to  an 
interesting  discovery  which  arose  out  of  these  experi- 
ments. The  lacticolor  variety  in  Great  Britain  is  a  south- 
ern form  and  is  not  known  to  occur  in  Scotland.  Matings 
were  made  between  wild  Scotch  females  and  lacticolor 
males.  The  families  resulting  from  such  matings  were 
precisely  the  same  as  those  from  lacticolor  males  and  FI 
females,  viz.  .grossulariata  males  and  lacticolor  females 
only.  We  are,  therefore,  forced  to  regard  the  constitution 
of  the  wild  grossulariata  female  as  identical  with  that  of 
the  FI  female,  i.e.  as  heterozygous  for  the  grossulariata 
factor  as  well  as  for  the  factor  for  femaleness.  Though 
from  a  region  where  lacticolor  is  unknown,  the  "pure" 
wild  grossulariata  female  is  nevertheless  a  permanent 
mongrel,  but  it  can  never  reveal  its  true  colours  unless  it  is 
mated  with  a  male  which  is  either  heterozygous  for  G  or 
pure  lacticolor.  And  as  all  the  wild  northern  males  are 


x  SEX  I05 

pure  for  the  grossulariata  character  this  can  never  happen 
in  a  state  of  nature. 

An  essential  feature  of  the  case  of  the  currant  moth  lies 
in  the  different  results  given  by  reciprocal  crosses.  Lacti- 
color female  X  grossulariata  male  gives  grossulariata  alone 
of  both  sexes.  But  grossulariata  female  X  lacticolor  male 
gives  only  grossulariata  males  and  lacticolor  females.  Such 
a  difference  between  reciprocal  crosses  has  also  been  found 
in  other  animals,  and  the  experimental  results,  though 
sometimes  more  complicated,  are  explicable  on  the  same 
lines.  An  interesting  case  in  which  three  factors  are 
concerned  has  been  recently  worked  out  in  poultry.  The 
Silky  breed  of  fowls  is  characterised  among  other  pecu- 
liarities by  a  remarkable  abundance  of  melanic  pigment. 
The  skin  is  dull  black,  while  the  comb  and  wattles  are  of 
a  deep  purple  colour  contrasting  sharply  with  the  white 
plumage  (PL  V.,  3) .  Dissection  shows  that  this  black  pig- 
ment is  widely  spread  throughout  the  body,  being  espe- 
cially marked  in  such  membranes  as  the  mesenteries,  the 
periosteum,  and  the  pia  mater  surrounding  the  brain. 
It  also  occurs  in  the  connective  tissues  among  the  muscles. 
In  the  Brown  Leghorn,  on  the  other  hand,  this  pigment 
is  not  found.  Reciprocal  crosses  between  these  two 
breeds  gave  a  remarkable  difference  in  result.  A  cross 
between  the  Silky  hen  and  the  Brown  Leghorn  cock  pro- 
duced FI  birds  in  which  both  sexes  exhibited  only  traces 
of  the  pigment.  On  casual  observation  they  might  have 


io6  MENDELISM  CHAP. 

passed  for  unpigmented  birds,  for  with  the  exception 
of   an   occasional   fleck   of   pigment   their   skin,    comb, 

and  wattles  were  as  clear 
Silky  Brown  Leghorn 

•    x     *  as  in  the  Brown  Leghorn 

(PL  V.,  i  and  4).     Dissec- 

r  ~~\  tion  revealed  the  presence 

1    of    a    slight    amount    of 


-i 1 1  internal   pigment.      Such 

i    d    <S     f    ?     ?     9'--F2    birds  bred  together  gave 

FlG-  I9'  some   offspring  with   the 

Scheme   illustrating  the  result  of  crossing  a 

Silky  hen  with    a    Brown    Leghorn    cock,    full     pigmentation     of    the 
Black  sex   signs    denote   deeply  pigmented 

birds,  and  light  sex  signs  those  without  pig-    Silky,    SOme    Without    any 
mentation.     The  light  signs  with  a  black  dot 

in  the  centre  denote  birds    with    a    small    pigment,  and  Others  shoW- 
amount  of  pigment. 

mg   different    degrees    of 

pigment.     None  of  the  F2  male  birds,  however,  showed 
the  full  deep  pigmentation  of  the  Silky. 

When,  however,  the  cross  was  made  the  other  way, 
viz.  Brown  Leghorn  hen  X  Silky  cock,  the  result  was 

different.    While  the 

Brown  Leghorn          Silky-. 
FI  male  birds  were  o    x    J 

almost  destitute   of 

pigment   as   in    the  p       x      3* F 

previous    cross,    the 

F!  hens,  on  the  other       , , , ,— L, , , , 

hand,    were    nearly  rf    rf    <$    d"     ¥     ?  -.?     9"-p* 

as  deeply  pigmented  FlG-  20- 

,  r"ii          Scheme  illustrating  the  result  of  crossing  a  Brown  Leg- 

aS      the     pure     Silky  horn  hen  with  a  Silky  cock  (cf.  Fig.  19). 


PLATE  V. 


i,  a,  F,  Cock  and  Hen,  ex  Brown  Leghorn  H»n   x    si  I-     C«    k: 
3,  Silky  Cock ;    4,  Hen  ex  Silky  Hen  x   Brown  Leghorn  Cock. 


x  SEX  107 

(PL  V. ,  2) .  The  male  Silky  transmitted  the  pigmentation, 
but  only  to  his  daughters.  Such  birds  bred  together 
gave  an  F2  generation  containing  chicks  with  the  full 
deep  pigment,  chicks  without  pigment,  and  chicks  with 
various  grades  of  pigmentation,  all  the  different  kinds  in 
both  sexes. 

In  analysing  this  complicated  case  many  other  different 
crosses  were  made,  but  for  the  present  it  will  be  sufficient 

(Brown  Leghorn)  Silky 

9       x      « 


( Brown  Legh.)  ^    X     ?  $    X     9  CBrown  Leghorn) 


e?  d    ?  9       rfe?dd?999 

FIG.  21. 

Scheme  to  illustrate  the  result  of  crossing  Ft  birds  (e.g.  Brown  Leghorn  X  Silky)  with  the 
pure  Brown  Leghorn. 

to  mention  but  one  of  these,  viz.  that  between  the  FI  birds 
and  the  pure  Brown  Leghorn.  The  cross  between  the  FI 
hen  and  the  Brown  Leghorn  cock  produced  only  birds 
with  a  slight  amount  of  pigment  and  birds  without  pig- 
ment. And  this  was  true  for  both  the  deeply  pigmented 
and  the  slightly  pigmented  types  of  FI  hen.  But  when 
the  FI  cock  was  mated  to  a  Brown  Leghorn  hen,  a  definite 
proportion  of  the  chicks,  one  in  eight,  was  deeply  pig- 
mented, and  these  deeply  pigmented  birds  were  always  fe- 
males (cf.  Fig.  21).  And  in  this  respect  all  the  FI  males 


io8  MENDELISM  CHAP. 

behaved  alike,  whether  they  were  from  the  Silky  hen  or 
from  the  Silky  cock.  We  have,  therefore,  the  paradox 
that  the  FI  hen,  though  herself  deeply  pigmented,  cannot 
transmit  this  condition  to  any  of  her  offspring  when  she 
is  mated  to  the  unpigmented  Brown  Leghorn,  but  that, 
when  similarly  mated,  the  FI  cock  can  transmit  this  pig- 
mented condition  to  a  quarter  of  his  female  offspring 
though  he  himself  is  almost  devoid  of  pigment. 

Now  all  these  apparently  complicated  results,  as  well 
as  many  others  to  which  we  have  not  alluded,  can  be  ex- 
pressed by  the  following  simple  scheme.  There  are  three 
factors  affecting  pigment,  viz.  (i)  a  pigmentation  fac- 
tor (P);  (2)  a  factor  which  inhibits  the  production  of 
pigment  (7) ;  and  (3)  a  factor  for  femaleness  (F),  for 
which  the  female  birds  are  heterozygous,  but  which  is  not 
present  in  the  males.  Further,  we  make  the  assumptions 
(a)  that  there  is  repulsion  between  F  and  /  in  the  female 
zygote  (Ffli),  and  (b)  that  the  male  Brown  Leghorn  is  ho- 
mozygous  for  the  inhibitor  factor  (7),  but  that  the  hen 
Brown  Leghorn  is  always  heterozygous  for  this  factor  just 
in  the  same  way  as  the  female  of  the  currant  moth  is  al- 
ways heterozygous  for  the  grossulariata  factor.  We  may 
now  proceed  to  show  how  this  explanation  fits  the  ex- 
perimental facts  which  we  have  given. 

The  Silky  is  pure  for  the  pigmentation  factor,  but  does 
not  contain  the  inhibitor  factor.  The  Brown  Leghorn,  on 
the  other  hand,  contains  the  inhibitor  factor,  but  not  the 


SEX 


109 


gives 
gametes 

FPi\ 
fPi  / 


gives 
gametes 


Upl 


FfPpli 


pigmentation  factor.  In  crossing  a  Silky  hen  with  a 
Brown  Leghorn  cock  we  are  mating  two  birds  of  the 
constitution  FfPPii  and  ffppll,  and  all  the  FI  birds  are 
consequently  heterozygous  for  both  P  and  7.  In  such 
birds  the  pigment  is  al- 

most  but  not  completely       "  ffPp"  [<*  '• 

suppressed,  and  as  both 
sexes  are  of  the  same 
constitution  with  regard 
to  these  two  factors  they 
are  both  of  similar  ap- 
pearance. 

In  the  reciprocal  cross, 
on  the  other  hand,  we 
are  mating  a  Silky  male 
(ffPPii)  with  a  Brown  Leghorn  hen  which  on  our  as- 
sumption is  heterozygous  for  the  inhibitor  factor  (7), 
and  in  constitution  therefore  is  Ffppli.  Owing  to 
the  repulsion  between  F  and  7  the  gametes  produced 
by  such  a  bird  are  Fpi  and  fpl  in  equal  numbers. 
All  the  gametes  produced  by  the  Silky  cock  are  fPi. 
Hence  the  constitution  of  the  FI  male  birds  produced  by 
this  cross  is  ffPpli  as  before,  but  the  female  birds  must 
be  all  of  the  constitution  FfPpii.  The  Silky  cock  trans- 
mits the  fully  pigmented  condition  to  his  daughters, 
because  the  gametes  of  the  Brown  Leghorn  hen  which  con- 
tain the  factor  for  femaleness  do  not  contain  the  inhibi- 


ffPpIi 

uu 

FIG.  22. 

Scheme  to  illustrate  the  nature  of  the  Y1  gener- 
ation from  the  Silky  hen  and  Brown  Leghorn 
cock  (cf.  Fig.  23). 


no  MENDELISM  CHAP. 

tory  factor  owing  to  the  repulsion  between  these  factors. 
The  nature  of  the  F2  generation  in  each  case  is  in  har- 
mony with  the  above  scheme.  As,  however,  it  serves  to 
illustrate  certain  points  in  connection  with  intermediate 
forms  we  shall  postpone  further  consideration  of  it  till  we 

_     discuss  these  matters,  and 
[9]FfPPIi  HPPiiW]     for  the  present  shall  limit 

gives  gives 

gametes  gametes  OUrselvCS    to  the  CXplana- 

F&\       v       /fpi  tion  of  tne  different  be- 

fnl    II  fPi 

haviour  of  the  FI  males 

, . — I !  and  females  when  crossed 

FfPpii  ffPpli  with  the  Brown  Leghorn. 

f*l  [<£!'  And,    first,    the    cross  of 

FlG-  23-  Brown  Leghorn  female  by 

Scheme  to  illustrate  the  nature  of  the  Fj  gen- 
eration from  the  Brown  Leghorn  hen  and      FI  male.      The  BrOWn  Leg- 
Silky  cock  (cf.  Fig.  22). 

horn  hen  is  on  our  hy- 
pothesis Ffppli,  and  produces  gametes  Fpi  and  fpi. 
The  FI  cock  is  on  our  hypothesis  ffPpli,  and  pro- 
duces in  equal  numbers  the  four  kinds.,  of  gametes 
fPI,  fPi,  fpi,  fpi.  The  result  of  the  meetin&of  these  two 
series  of  gametes  is  given  in  Fig.  24.  Of  the  eight  differ- 
ent kinds  of  zygote  formed  only  one  contains  P  in  the 
absence  of  /,  and  this  is  a  female.  The  result,  as  we  have 
already  seen,  is  in  accordance  with  the  experimental  facts. 
On  the  other  hand,  the  Brown  Leghorn  cock  is  on  our 
hypothesis//"/?/?//.  All  his  gametes  consequently  contain 
the  inhibitor  factor,  and  when  he  is  mated  with  an  FI 


SEX 


in 


Fpi 
fPI 

9 

Fpi 
fPi 

* 

Fpi 
fpl 

9 

Fpi 
fpi 

9 

fpl 
fPI 

I 

fpi 

fPi 
df 

fPi 
fPi 

3 

fpi 
fpi 

cf 

FIG.  24. 

f     ji  -p  Diagram  showing  the  nature  of  the  offspring  from  a 

:OWn        Brown  Leghorn  hen  and  an  Fl  cock  bred  from  Silky 
hen  X  Brown  Leghorn  cock,  or  vice  versa. 


hen  all  the  zygotes  produced  must  contain  7.  None  of  his 
offspring,  therefore,  can  be  fully  pigmented,  for  this  con- 
dition only  occurs  in  the  absence  of  the  inhibitor  factor 
among  zygotes  which 
are  either  homozy- 
gous  or  heterozygous 
for  P. 

The  interpretation 
of  this  case  turns 
upon  the  constitu- 
tion 

Leghorn  hen,  upon 
her  heterozygous  condition  with  regard  to  the  two  factors 
F  and  7,  and  upon  the  repulsion  that  occurs  between  them 
when  the  gametes  are  formed.  Through  an  independent 
set  of  experiments  this  view  of  the  nature  of  the  Brown 
Leghorn  hen  has  been  confirmed  in  an  interesting  way. 
There  are  fowls  which  possess  neither  the  factor  for  pig- 
ment nor  the  inhibitory  factor,  which  are  in  constitution 
ppii.  Such  birds  when  crossed  with  the  Silky  give  dark 
pigmented  birds  of  both  sexes  in  FI,  and  the  F2  genera- 
tion consists  of  pigmented  and  unpigmented  in  the  ratio 
3:1.  Now  a  cock  of  such  a  strain  crossed  with  a  Brown 
Leghorn  hen  should  give  only  completely  unpigmented 
birds.  But  if,  as  we  have  supposed,  the  Brown  Leghorn 
hen  is  producing  gametes  Fpi  and  fpl,  the  male  birds 
produced  by  such  a  cross  should  be  heterozygous  for  7, 


112 


MENDELISM 


CHAP. 


i.e.  in  constitution  ffppli,  while  the  hen  birds,  though 
identical  in  appearance  so  far  as  absence  of  pigmentation 
goes,  should  not  contain  this  factor  but  should  be  con- 
stitutionally Ffppii.  Crossed  with  the  pure  Silky,  the  FI 

[9]  Ffppli  ffppii 

gives  gives 

gametes  gametes 


[rf]ffPPii       [9]  Ffppii 

gives  gives 

gametes  gametes 


Ffppli  [c?]        FfPPiiff] 


gves 
gametes 


gves 
gametes 


fPi 


fpi 


fp 


FfPpii 


ffPpii    FfPpli     FfPpii      ffPpli      ffPpii 


FIG.  25. 

Scheme  to  illustrate  the  heterozygous  nature  of  the  pure  Brown  Leghorn  hen.    For 
explanation  see  text. 

birds  of  opposite  sexes  should  give  an  entirely  different 
result.  For  while  the  hens  should  give  only  deeply  pig- 
mented  birds  of  both  sexes,  the  cocks  should  give  equal 
numbers  of  deeply  pigmented  and  slightly  pigmented 
birds  (cf.  Fig.  25).  These  were  the  results  which  the  ex- 
periment actually  gave,  thus  affording  strong  confirma- 
tion of  the  view  which  we  have  been  led  to-  take  of  the 
Brown  Leghorn  hen.  Essentially  the  poultry  case  is  that 
of  the  currant  moth.  It  differs  in  that  the  factor  which 


x  SEX  113 

repels  femaleness  produces  no  visible  effect,  and  its  pres- 
ence or  absence  can  only  be  determined  by  the  introduc- 
tion of  a  third  factor,  that  for  pigmentation. 

This  conception  of  the  nature  of  the  Brown  Leghorn 
hen  leads  to  a  curious  paradox.  We  have  stated  that 
the  Silky  cock  transmits  the  pigmented  condition,  but 
transmits  it  to  his  daughters  only.  Apparently  the  case 
is  one  of  unequal  transmission  by  the  father.  Actually, 
as  our  analysis  has  shown,  it  is  one  of  unequal  transmis- 
sion by  the  mother,  the  father's  contribution  to  the  off- 
spring being  identical  for  each  sex.  The  mother  trans- 
mits to  the  daughters  her  dominant  quality  of  femaleness, 
but  to  balance  this,  as  it  were,  she  transmits  to  her  sons 
another  quality  which  her  daughters  do  not  receive.  It  is 
a  matter  of  common  experience  among  human  families 
that  in  respect  to  particular  qualities  the  sons  tend  to  re- 
semble their  mothers  more  than  the  daughters  do,  and  it 
is  not  improbable  that  such  observations  have  a  real 
foundation  for  which  the  clue  may  be  provided  by  the 
Brown  Leghorn  hen. 

Nor  is  this  the  only  reflection  that  the  Brown  Leghorn 
suggests.  Owing  to  the  repulsion  between  the  factors  for 
femaleness  and  for  pigment  inhibition,  it  is  impossible  by 
any  form  of  mating  to  make  a  hen  which  is  homozygous 
for  the  inhibitor  factor.  She  has  bartered  away  for  fe- 
maleness the  possibility  of  ever  receiving  a  double  dose  of 
this  factor.  We  know  that  in  some  cases,  as,  for  example, 


ii4  MENDELISM  CHAP,  xi 

that  of  the  blue  Andalusian  fowl,  the  qualities  of  the  in- 
dividual are  markedly  different  according  as  to  whether 
he  or  she  has  received  a  single  or  a  double  dose  of  a  given 
factor.  It  is  not  inconceivable  that  some  of  the  quali- 
ties in  which  a  man  differs  from  a  woman  ar  founded 
upon  a  distinction  of  this  nature.  Certain  qualities  oi 
intellect,  for  example,  may  depend  upon  the  existence  in 
the  individual  of  a  double  dose  of  some  factor  which  is  re- 
pelled by  femaleness.  If  this  is  so,  and  if  woman  is  bent 
upon  achieving  the  results  which  such  qualities  of  intellect 
imply,  it  is  not  education  or  training  that  will  help  her. 
Her  problem  is  to  get  the  factor  on  which  the  quality 
depends  into  an  ovum  that  carries  also  the  factor  for 
femaleness. 


CHAPTER  XI 

SEX  (continued) 

THE  cases  which  we  have  considered  in  the  last  chap- 
ter belong  to  a  group  in  which  the  peculiarities  of  in- 
heritance are  most  easily  explained  by  supposing  that  the 
female  is  heterozygous  for  some  factor  that  is  not  found  in 
the  male.  Femaleness  is  an  additional  character  super- 
posed upon  a  basis  of  maleness,  and  as  we  imagine  that 
there  is  a  separate  factor  for  each  the  full  constitutional 
formula  for  a  female  is  FfMM,  and  for  a  male  ffMM. 
Both  sexes  are  homozygous  for  the  male  element,  and  the 
difference  between  them  is  due  to  the  presence  or  absence 
of  the  female  element  F. 

There  are,  however,  other  cases  for  which  the  explana- 
tion will  not  suffice,  but  can  be  best  interpreted  on  the 
view  that  the  male  is  heterozygous  for  a  factor  which  is 
not  found  in  the  female.  Such  a  case  is  that  recently 
described  by  Morgan  in  America  for  the  pomace  fly 
(Drosophila  ampelophila).  Normally  this  little  insect 
has  a  red  eye,  but  white  eyed  individuals  are  known  to 
occur  as  rare  sports.  Red  eye  is  dominant  to  white. 
In  their  relation  to  sex  the  eye  colours  of  the  pomace  fly 


u6  MENDELISM  CHAP. 

are  inherited  on  the  same  lines  as  the  grossulariata  and 
lacticolor  patterns  of  the  currant  moth,  but  with  one  essen- 
tial difference.  The  factor  which  repels  the  red-eye  factor 
is  in  this  case  to  be  found  in  the  male,  and  here  conse- 
quently it  is  the  male  which  must  be  regarded  as  hetero- 
zygous for  a  sex  factor  that  is  lacking  in  the  female. 

In  order  to  bring  these  cases  and  others  into  line  an 
interesting  suggestion  has  recently  been  put  forward  by 
Bateson.     On  this  suggestion  each  sex  is  heterozygous  for 
its  own  sex  factor  only,  and  does  not  contain  the  factor 
proper  to  the  opposite  sex.     The  male  is  of  the  constitu- 
tion, Mmff   and    the    female 
Mmff         Ffmm  Ffmm.       Each    sex    produces 

gives 

gametes  two  sorts  of  gametes,  Mf  and 

fm  ^productive      mf  in  the  case  of  the  male,  and 

Fmj  fertilisations      _,          .       .        .  ,     .        -          . 

^unproductive    F™>  ^  1R  that  °f  the  female' 
fm  /fertilisations     But  Qn  flfo  vjew  &  further  SUp- 

position  is  necessary.     If  each 

of  the  two  kinds  of  spermatozoa  were  capable  of  fertilis- 
ing each  of  the  two  kinds  of  ova,  we  should  get  individuals 
of  the  constitution  MmFf  and  mmff,  as  well  as  the  normal 
males  and  females,  Mmff  and  Ffmm.  As  the  facts  of  or- 
dinary bisexual  reproduction  afford  us  no  grounds  for 
assuming  the  existence  of  these  two  classes  of  individuals, 
whatever  they  may  be,  we  must  suppose  that  fertilisation 
is  productive  only  between  the  spermatozoa  carrying 
M  and  the  ova  without  F,  or  between  the  spermatozoa 


xi  SEX  117 

without  M  and  the  ova  containing  F.  In  other  words  we 
must  on  this  view  suppose  that  fertilisations  between 
certain  forms  of  gametes,  even  if  they  can  occur,  are  in- 
capable of  giving  rise  to  zygotes  with  the  capacity  for 
further  development.  If  we  admit  this  supposition,  the 
scheme  just  given  will  cover  such  cases  as  those  of  the 
currant  moth  and  the  fowl,  equally  as  well  as  that  of  the 
pomace  fly.  In  the  former  there  is  repulsion  between 
either  the  grossulariata  factor  and  F,  or  else  between  the 
pigment  inhibitor  factor  and  F,  while  in  the  latter  there 
is  repulsion  between  the  factor  for  red  eye  and  M . 

Whatever  the  merits  or  demerits  of  such  a  scheme  it 
certainly  does  offer  an  explanation  of  a  peculiar  form  of 
sex  limited  inheritance  in  man.  *  * 

It  has  long  been  a  matter  of  com-  | 

mon  knowledge  that  colour-blind-   9  #  ?     x  <* 

ness  is  much  more  common  among    7"^         '      '     '      ' 
men  than  among  women,  and  also  FIG  ^ 

that  Unaffected  WOmen  Can  tranS-  Scheme  to  illustrate  the  probable 

.         .                     .      .                              .           ..  mode  of   inheritance  of  colour- 

mit     it     tO     their     SOnS.       At     first  blindness.    The   dark  signs  re- 

...                             .                               ,M  present     affected      individuals. 

Sight       the      Case      IS      not      Unlike  A  black  dot  in  the  centre  de- 

,                   ,  notes  an  unaffected  female  who 

that      OI    the       Sheep,      Where      the  is  capable  of  transmitting  the 

.                .        ,                           .                              , ,  condition  to  her  sons. 

horned   character    is   apparently 

dominant  in  the  male  but  recessive  in  the  female. 
The  hypothesis  that  the  colour-blind  condition  is 
due  to  the  presence  of  an  extra  factor  as  compared  with 
the  normal,  and  that  a  single  dose  of  it  will  produce 


n8  MENDELISM  CHAP. 

colour-blindness  in  the  male  but  not  in  the  female,  will 
cover  a  good  many  of  the  observed  facts  (cf.  Fig.  26). 
Moreover,  it  serves  to  explain  the  remarkable  fact  that  all 
the  sons  of  colour-blind  women  are  also  colour-blind.  For 
a  woman  cannot  be  colour-blind  unless  she  is  homozygous 
for  the  colour-blind  factor,  in  which  case  all  her  children 
must  get  a  single  dose  of  it  even  if  she  marries  a  normal 
male.  And  this  is  sufficient  to  produce  colour-blindness 
in  the  male,  though  not  in  the  female. 

But  there  is  one  notable  difference  in  this  case  as  com- 
pared with  that  of  the  sheep.  When  crossed  with  pure 
hornless  ewes  the  heterozygous  horned  ram  transmits  the 
horned  character  to  half  his  male  offspring  (cf.  p.  71). 
But  the  heterozygous  colour-blind  man  does  not  behave 
altogether  like  a  sheep,  for  he  apparently  does  not  trans- 
mit the  colour-blind  condition  to  any  of  his  male  offspring. 
If,  however,  we  suppose  that  the  colour-blind  factor  is 
repelled  by  the  factor  for  maleness,  the  amended  scheme 
will  cover  the  observed  facts.  For,  denoting  the  colour- 
blind factor  by  X,  the  gametes  produced  by  the  colour- 
blind male  are  of  two  sorts  only,  viz.  Mfx  and  m/X. 
If  he  marries  a  normal  woman  (Ffmmxx) ,  the  spermatozoa 
Mfx  unite  with  ova  fmx  to  give  normal  males,  while  the 
spermatozoa  mfX  unite  with  ova  Fmx  to  give  females 
which  are  heterozygous  for  the  colourblind  factor. 
These  daughters  are  themselves  normal,  but  transmit  the 
condition  to  about  half  their  sons. 


xi  SEX  119 

The  attempt  to  discover  a  simple  explanation  of  the 
nature  of  sex  has  led  us  to  assume  that  certain  combina- 
tions between  gametes  are  incapable  of  giving  rise  to  zy- 
gotes  which  can  develop  further.  In  the  various  cases 
hitherto  considered  there  is  no  reason  to  suppose  that 
anything  of  the  sort  occurs,  or  that  the  different  gametes 
are  otherwise  than  completely  fertile  one  with  another. 
One  peculiar  case,  however,  has  been  known  for  several 
years  in  which  some  of  the  gametes  are  apparently  incapa- 
ble of  uniting  to  produce  offspring.  Yellow  in  the  mouse 
is  dominant  to  agouti,  but  hitherto  a  homozygous  yellow 
has  never  been  met  with.  The  yellows  from  families 
where  only  yellows  and  agoutis  occur  produce,  when  bred 
together,  yellows  and  agoutis  in  the  ratio  2:1.  If  it  were 
an  ordinary  Mendelian  case  the  ratio  should  be  3  :  i ,  and 
one  out  of  every  three  yellows  so  bred  should  be  homo- 
zygous and  give  only  yellows  when  crossed  with  agouti. 
But  Cuenot  and  others  have  shown  that  all  of  the  yellows 
are  heterozygous,  and  when  crossed  with  agoutis  give 
both  yellows  and  agoutis.  We  are  led,  therefore,  to  sup- 
pose that  an  ovum  carrying  the  yellow  factor  is  unpro- 
ductive if  fertilised  by  a  spermatozoon  which  also  bears 
this  factor.  In  this  way  alone  does  it  seem  possible  to 
explain  the  deficiency  of  yellows  and  the  absence  of  homo- 
zygous ones  in  the  families  arising  from  the  mating  of 
yellows  together.  At  present,  however,  it  remains  the 
only  definite  instance  among  animals  in  which  we  have  • 


120  MENDELISM  CHAP. 

grounds  for  assuming  that  anything  in  the  nature  of 
unproductive  fertilisation  takes  place.1 

If  we  turn  from  animals  to  plants  we  find  a  more  com- 
plicated state  of  affairs.  Generally  speaking,  the  higher 
plants  are  hermaphrodite,  both  ovules  and  pollen  grains 
occurring  on  the  same  flower.  Some  plants,  however, 
like  most  animals,  are  of  separate  sexes,  a  single  plant 
bearing  only  male  or  female  flowers.  In  other  plants  the 
separate  flowers  are  either  male  or  female,  though  both 
are  borne  on  the  same  individual.  In  others,  again,  the 
conditions  are  even  more  complex,  for  the  same  plant 
may  bear  flowers  of  three  kinds,  viz.  male,  female,  and 
hermaphrodite.  Or  it  may  be  that  these  three  forms 
occur  in  the  same  species  but  in  different  individuals 
-female  and  hermaphrodites  in  one  species;  males, 
females,  and  hermaphrodites  in  another.  One  case, 
however,  must  be  mentioned  as  it  suggests  a  possibility 
which  we  have  not  hitherto  encountered.  In  the  com- 
mon English  bryony  (Bryonia  dioica}  the  sexes  are  sep- 
arate, some  plants  having  only  male  and  others  only  fe- 
male flowers.  In  another  European  species,  B.  alba,  both 
male  and  female  flowers  occur  on  the  same  plant. 
Correns  crossed  these  two  species  reciprocally,  and  also 
fertilised  B.  dioica  by  its  own  male  with  the  following 
results : — 

1  For  the  most  recent  discussion  of  this  peculiar  case  the  reader  is  re- 
ferred to  Professor  Castle's  paper  in  Science,  December  16,  1910. 


xi  SEX  121 

dioica  £   x  dioica  $  gave  9    9  and  $   $ 

„  x  alba      $      ,,      $    <?  only 

alba     $   x  dioica  £     „      $    $  and  $   $ . 

The  point  of  chief  interest  lies  in  the  striking  difference 
'shown  by  the  reciprocal  crosses  between  dioica  and  alba. 
Males  appear  when  alba  is  used  as  the  female  parent  but 
not  when  the  female  dioica  is  crossed  by  male  alba.  It  is 
possible  to  suggest  more  than  one  scheme  to  cover  these 
facts,  but  we  may  confine  ourselves  here  to  that  which 
seems  most  in  accord  with  the  general  trend  of  other 
cases.  We  will  suppose  that  in  dioica  femaleness  is 
dominant  to  maleness,  and  that  the  female  is  heterozygous 
for  this  additional  factor.  In  this  species,  then,  the  fe- 
male produces  equal  numbers  of  ovules  with  and  without 
the  female  factor,  while  this  factor  is  absent  in  all  the 
pollen  grains.  Alba$  x  dioica S  gives  the  same  result  as 
dioica  $  X  dioica  $ ,  and  we  must  therefore  suppose  that 
alba  produces  male  and  female  ovules  in  equal  numbers. 
Alba 3  X  dioica  $,  however,  gives  nothing  but  females. 
Unless,  therefore,  we  assume  that  there  is  selective  fer- 
tilisation we  must  suppose  that  all  the  pollen  grains  of 
alba  carry  the  female  factor  —  in  othef  words,  that  so  far 
as  the  sex:  factors  are  concerned  there  is  a  difference  be- 
tween the  ovules  and  pollen  grains  borne  by  the  same 
plant.  Unfortunately  further  investigation  of  this  case 
is  rendered  impossible  owing  to  the  complete  sterility  of 
the  FI  plants. 


122  MENDELISM  CHAP. 

That  the  possibility  of  a  difference  between  the  ovules 
and  pollen  grains  of  the  same  individual  must  be  taken 
into  account  in  future  work  there  is  evidence  from  quite 
a  different  source.  The  double  stock  is  an  old  horti- 
cultural favourite,  and  for  centuries  it  has  been  known 
that  of  itself  it  sets  no  seed,  but  must  be  raised  from 
special  strains  of  the  single  variety.  "You  must  under- 
stand withall,"  wrote  John  Parkinson  of  his  gilloflowers,1 
"that  those  plants  that  beare  double  flowers,  doe  beare 
no  seed  at  all  ...  but  the  onely  way  to  have  double 
flowers  any  yeare  is  to  save  the  seedes  of  those  plants  of 
this  kinde  that  beare  single  flowers,  for  from  that  seede 
will  rise  some  that  will  beare  single,  and  some  double 
flowers."  With  regard  to  the  nature  of  these  double- 
throwing  strains  of  singles,  Miss  Saunders  has  recently 
brought  out  some  interesting  facts.  She  crossed  the 
double- thro  wing  singles  with  pure  singles  belonging  to 
strains  in  which  doubles  never  occur.  The  cross  was 
made  both  ways,  and  in  both  cases  all  the  FI  plants  were 
single.  A  distinction,  however,  appeared  when  a  further 
generation  was  raised  from  the  FI  plants.  All  the  FI 
plants  from  the  pollen  of  the  double-throwing  single  be- 
haved like  double-throwing  singles,  but  of  the  FI  plants 
from  the  ovules  of  the  double  throwers  some  behaved 
as  double  throwers,  and  some  as  pure  singles.  We  are 
led  to  infer,  therefore,  that  the  ovules  and  pollen  grains 

1  Paradisus  Terrestris,  London,  1629,  p.  261. 


xi  SEX 


123 


of  the  double  throwers,  though  both  produced  by  the 
same  plant,  differ  in  their  relation  to  the  factor  (or  factors) 
for  doubleness.  Doubleness  is  apparently  carried  by  all 
the  pollen  grains  of  such  plants,  but  only  by  some  of  the 
ovules.  Though  the  nature  of  doubleness  in  stocks  is 


FIG.  27. 
Single  and  double  stocks  raised  from  the  same  single  parent. 

not  yet  clearly  understood,  the  facts  discovered  by  Miss 
Saunders  suggest  strongly  that  the  ovules  and  pollen 
grains  of  the  same  plant  may  differ  in  their  transmitting 
properties,  probably  owing  to  some  process  of  segregation 
in  the  growing  plant  which  leads  to  an  unequal  distri- 
bution of  some  or  other  factors  to  the  cells  which  give 
rise  to  the  ovules  as  compared  with  those  from  which 


124  MENDELISM  CHAP,  xi 

the  pollen  grains  eventually  spring.  Whether  this  may 
turn  out  to  be  the  true  account  or  not,  the  possibility 
must  not  be  overlooked  in  future  work. 

From  all  this  it  is  clear  enough  that  there  is  much  to 
be  done  before  the  problem  of  sex  is  solved  even  so  far  as 
the  biologist  can  ever  expect  to  solve  it.  The  possibilities 

Single 


Single  Double 


Pollen  of     Y    «_£          n-ii—        x.  Ovule  of 
pure  single 


Single         Single  Single 

I          I ,  I- 


I 
Single         Single      Double  Single          Double 


I  I  I  I 

Single         Single     Double  Single          Double 

are  many,  and  many  a  fresh  set  of  facts  is  needed  before 
we  can  hope  to  decide  among  them.  Yet  the  occasional 
glimpses  of  clear-cut  and  orderly  phenomena,  which  Men- 
delian  spectacles  have  already  enabled  us  to  catch,  offer 
a  fair  hope  that  some  day  they  may  all  be  brought  into 
focus,  and  assigned  their  proper  places  in  a  general 
scheme  which  shall  embrace  them  all.  Then,  though 
not  till  then,  will  the  problem  of  the  nature"  of  sex  pass 
from  the  hands  of  the  biologist  into  those  of  the  physicist 
and  the  chemist. 


CHAPTER   XII 

INTERMEDIATES 

So  far  as  we  have  gone  we  have  found  it  possible  to 
express  the  various  characters  of  animals  and  plants  in 
terms  of  definite  factors  which  are  carried  by  the  gametes, 
and  are  distributed  according  to  a  definite  scheme. 
Whatever  may  be  the  nature  of  these  factors  it  is  possible 
for  purposes  of  analysis  to  treat  them  as  indivisible  en- 
tities which  may  or  may  not  be  present  in  any  given 
gamete.  When  the  factor  is  present  it  is  present  as  a 
whole.  The  visible  properties  developed  by  a  zygote  in  the 
course  of  its  growth  depend  upon  the  nature  and  variety 
of  the  factors  carried  in  by  the  two  gametes  which  went 
to  its  making,  and  to  a  less  degree  upon  whether  each 
factor  was  brought  in  by  both  gametes  or  by  one  only. 
If  the  given  factor  is  brought  in  by  one  gamete  only,  the 
resulting  heterozygote  may  be  more  or  less  intermediate 
between  the  homozygous  form  with  a  double  dose  of  the 
factor  and  the  homozygous  form  which  is  entirely  des- 
titute of  the  factor.  Cases  in  point  are  those  of  the 
primula  flowers  and  the  Andalusian  fowls.  Nevertheless 
these  intermediates  produce  only  pure  gametes,  as  is 

125 


126  MENDELISM  CHAP. 

shown  by  the  fact  that  the  pure  parental  types  appear  in 
a  certain  proportion  of  their  offspring.  In  such  cases  as 
these  there  is  but  a  single  type  of  intermediate,  and  the 
simple  ratio  in  which  this  and  the  two  homozygous  forms 
appear  renders  the  interpretation  obvious.  But  the 
nature  of  the  F2  generation  may  be  much  more  complex, 
and,  where  we  are  dealing  with  factors  which  interact 
upon  one  another,  may  even  present  the  appearance  of  a 
series  of  intermediate  forms  grading  from  the  condition 
found  in  one  of  the  original  parents  to  that  which  occurred 
in  the  other.  As  an  illustration  we  may  consider  the 
cross  between  the  Brown  Leghorn  and  Silky  fowls  which 
we  have  already  dealt  with  in  connection  with  the  inheri- 
tance of  sex.  The  offspring  of  a  Silky  hen  mated  with  a 
Brown  Leghorn  are  in  both  sexes  birds  with  but  a  trace  of 
the  Silky  pigmentation.  But  when  such  birds-  are  bred 
together  they  produce  a  generation  consisting  of  chicks  as 
deeply  pigmented  as  the  original  Silky  parent,  chicks 
devoid  of  pigment  like  the  Brown  Leghorn,  and  chicks  in 
which  the  pigmentation  shows  itself  in  a  variety  of  inter- 
mediate stages.  Indeed  from  a  hundred  chicks  bred  in 
this  way  it  would  be  possible  to  pick  out  a  number  of  indi- 
viduals and  arrange  them  in  an  apparently  continuous 
series  of  gradually  increasing  pigmentation,  with  the 
completely  unpigmented  at  one  end  and  the  most  deeply 
pigmented  at  the  other.  Nevertheless,  the  case  is  one  in 
which  complete  segregation  of  the  different  factors  takes 


XII 


INTERMEDIATES 


127 


place,  and  the  apparently  continuous  series  of  interme- 
diates is  the  result  of  the  interaction  of  the  different  factors 
upon  one  another.  The  constitution  of  the  FI  $  is  a  //Ppli, 
and  such  a  bird  produces  in  equal  numbers  the  four  sorts 
of  gametes  /PI,  /Pi,  fpl,  /pi.  The  constitution  of  the  FI  $ 
in  this  case  is  F/PpIi.  Owing  to  the  repulsion  between 
F  and  7  she  produces  the  four  kinds  of  gametes  FPi,  Fpi, 
/PI,  /pi,  and  produces  them  in  equal  numbers.  The 
result  of  bringing  two 
such  series  of  gametes 
together  is  shown  in 
Fig.  28.  Out  of  the 
sixteen  types  of  zygote 
formed  one  (F/PPii) 
is  homozygous  for  the 
pigmentation  factor, 
and  does  not  contain 
the  inhibitor  factor. 
Such  a  bird  is  as  deeply 
pigmented  as  the  pure  FIG.  28. 

^illrv     nsrpnt  Twn     Diagram  to  illustrate  the  nature  and  composition  of 

OllKy     pdlCIlL.  L  WU,        the  F2  generations  arising  from  the  cross  of  Silky 

again,  contain  a  hen with Brown Leghorn cock" 
single  dose  of  P  in  the  absence  of  7.  These  are 
nearly  as  dark  as  the  pure  Silky.  Four  '  zygotes  are 
destitute  of  P,  though  they  may  or  may  not  contain  7. 
These  birds  are  completely  devoid  of  pigment  like  the 
Brown  Leghorn.  The  remaining  nine  zygotes  show 


FPi 
fPI 

* 

FPi 
fPi 

f 

FPi 
fpl 

* 

FPi 
fpi 

? 

Fpi 
fPI 

9 

Fpi 
fPi 

9 

Fpi 
fpl 

9 

Fpi 
fpi 

9 

fPI 
fPI 

I 

fpi 

fPi 
6 

fpi 
fpi 

4 

fpi 
fpi 

4 

fpl 
fPI 

I 

fpl 
fPi 

<$ 

fPi 
fpi 

4 

fPi 
fpi 

4 

128  MENDELISM  CHAP. 

various  combinations  of  the  two  factors  P  and  /,  being 
either  PPIi,  PPII,  PpII,  or  Ppli,  and  in  each  of  these 
cases  the  pigment  is  more  or  less  intense  according  to  the 
constitution  of  the  bird.  Thus  a  bird  of  the  constitution 
PPIi  approaches  in  pigmentation  a  bird  of  the  constitu- 
tion Ppii,  while  a  bird  of  the  constitution  PpII  has  but 
little  more  pigment  than  the  unpigmented  bird.  In  this 
way  we  have  seven  distinct  grades  of  pigmentation,  and 
the  series  is  further  complicated  by  the  fact  that  these 
various  grades  exhibit  a  rather  different  amount  of  pig- 
mentation according  as  they  occur  in  a  male  or  a  female 
bird,,  for,  generally  speaking,  the  female  of  a  given  grade 
exhibits  rather  more  pigment  than  the  corresponding 
male.  The  examination  of  a  number  of  birds  bred  in  this 
way  might  quite  well  suggest  that  in  this  case  we  were 
dealing  with  a  character  which  could  break  ,up,  as  it 
were,  to  give  a  continuous  series  of  intergrading  forms 
between  the  two  extremes.  With  the  constant  handling 
of  large  numbers  it  becomes  possible  to  recognise  most  of 
the  different  grades,  though  even  so  it  is  possible  to  make 
mistakes.  Nevertheless,  as  breeding  tests  have  amply 
shown,  we  are  dealing  with  but  two  interacting  factors 
which  segregate  cleanly  from  one  another  according  to 
the  strict  Mendelian  rule.  The  approach  to  continuity 
in  variation  exhibited  by  the  F2  generation  depends  upon 
the  fact  that  these  two  factors  interact  upon  one  another, 
and  to  different  degrees  according  as  the  zygote  is  for  one 


xii  INTERMEDIATES  129 

or  other  or  both  of  them  in  a  homozygous  or  a  hetero- 
zygous state.  Moreover,  certain  of  these  intermediates 
will  breed  true  to  an  intermediate  condition  of  the  pig- 
mentation. A  male  of  the  constitution  ffPPII  when 
bred  with  females  of  the  constitution  FfPPIi  will  produce 
only  males  like  itself  and  females  like  the  maternal  parent. 
We  have  dealt  with  this  case  in  some  detail,  because  the 
existence  of  families  showing  a  series  of  intermediate  stages 
between  two  characters  has  sometimes  been  brought 
forward  in  opposition  to  the  view  that  the  characters  of 
organisms  depend  upon  specific  factors  which  are  trans- 
mitted according  to  the  Mendelian  rule.  But,  as  this 
case  from  poultry  shows  clearly,  neither  the  existence  of 
such  a  continuous  series  of  intermediates,  nor  the  fact 
that  some  of  them  may  breed  true  to  the  intermediate 
condition,  are  incompatible  with  the  Mendelian  principle 
of  segregation. 

In  connection  with  intermediates  a  more  cogent  ob- 
jection to  the  Mendelian  view  is  the  case  of  the  first  cross 
between  two  definite  varieties  thenceforward  breeding 
true.  The  case  that  will  naturally  occur  to  the  reader  is 
that  of  the  mulatto,  which  results  from  the  cross  between 
the  negro  and  the  white.  According  to  general  opinion, 
these  mulattos,  of  intermediate  pigmentation,  continue  to 
produce  mulattos.  Unfortunately  this  interesting  case  has 
never  been  critically  investigated,  and  the  statement  that 
the  mulatto  breeds  true  rests  almost  entirely  upon  in- 


130 


MENDELISM 


CHAP. 


formation  that  is  general  and  often  vague.  It  may  be 
that  the  inheritance  of  skin  pigmentation  in  this  instance 
is  a  genuine  exception  to  the  normal  rule,  but  at  the  same 
time  it  must  not  be  forgotten  that  it  may  be  one  in  which 
several  interacting  factors  are  concerned,  and  that  the 


9 


1                1                1                Jill 

<<$<$$       ?*c?G?   ?    ? 
U                  r-jJ-,' 

QXG?     fxj      0x5 

wji^1 


vW  w    Several  children 

all   0    or    O 

FIG.  29.  , 

Pedigree  of  a  family  which  originated  from  a  cross  between  a  Hindu  and  a  European. 
Black  signs  denote  individuals  as  dark  as  average  Hindus.  Plain  "signs  denote  quite 
fair  members,  while  those  with  a  dot  in  the  centre  are  intermediate. 

pure  white  and  the  pure  black  are  the  result  of  combina- 
tions which  from  their  rarity  are  apt  to  be  overlooked. 
But  until  we  are  in  possession  of  accurate  information  it 
is  impossible  to  pronounce  definitely,  upon  the  nature  of 
the  inheritance  in  this  case. 


xii  INTERMEDIATES  131 

On  the  other  hand,  from  the  cross  between  the  darkly 
pigmented  Eastern  races  and  the  white  segregation  seems 
to  occur  in  subsequent  generations.  Families  are  to  be 
found  in  which  one  parent  is  a  pure  white,  while  the  other 
has  arisen  from  the  cross  between  the  dark  and  light  in 
the  first  or  some  subsequent  generation.  Such  families 
may  contain  children  indistinguishable  from  pure  blonds  as 
well  as  children  of  very  dark  and  of  intermediate  shades. 
As  an  example,  I  may  give  the  following  pedigree,  which 
was  kindly  communicated  to  me  by  an  Anglo-Indian 
friend  (Fig.  29).  The  family  had  resided  in  England  for 
several  generations,  so  that  in  this  case  there  was  no  ques- 
tion of  a  further  admixture  of  black.  Most  noticeable  is 
the  family  produced  by  a  very  dark  lady  who  had  mar- 
ried a  white  man.  Some  of  the  children  were  interme- 
diate in  colour,  but  two  were  fair  whites  and  two  were  dark 
as  dark  Hindus.  This  sharp  segregation  or  splitting  out 
of  blacks  and  whites  in  addition  to  intermediates  strongly 
suggests  that  the  nature  of  the  inheritance  is  Mendelian, 
though  it  may  be  complicated  by  the  existence  of  several 
factors  which  may  also  react  upon  one  another.  Nor 
must  it  be  forgotten  that  in  so  far  as  these  different  factors 
are  concerned  the  whites  themselves  may  differ  in  con- 
stitution without  showing  any  trace  of  it  in  their  appear- 
ance. Before  the  case  can  be  regarded  as  settled  all  these 
different  possibilities  will  have  to  be  definitely  tested. 
With  the  dark  Eastern  races  as  with  the  negro  we  cannot 


i32  MENDELISM  CHAP. 

hope  to  come  to  any  conclusion  until  we  have  evidence 
collected  by  critical  and  competent  observers. 

Though  for  the  present  we  must  regard  the  case  of  the 
negro  as  not  proven,  there  are  nevertheless  two  others  in 
which  the  heredity  would  appear  not  to  follow  the  Men- 
delian  rule.  Castle  in  America  crossed  the  lop-eared 
rabbit  with  the  normal  form,  and  found  that  the  FI  ani- 
mals were  intermediate  with  respect  to  their  ears. 
And  subsequent  experiment  showed  that,  on  the  whole, 
they  bred  true  to  this  intermediate  condition.  The  other 
case  relates  to  Lepidoptera.  The  speckled  wood  butter- 
fly (Pararge  egerid)  has  a  southern  form  which  differs 
from  the  northern  one  in  the  greater  brightness  and  depth 
of  its  yellow-brown  markings.  The  northern  form  is 
generally  distinguished  as  var.  egeriades.  Bateson 
crossed  the  southern  form  from  the  south  of  France  with 
the  paler  British  form,  and  found  that  the  offspring  were 
more  or  less  intermediate  in  colour,  and  that  in  subse- 
quent generations  the  parental  types  did  not  recur. 
These  cases  at  present  stand  alone.  It  is  possible  that 
further  research  may  reveal  complications  which  mask  or 
interfere  with  an  underlying  process  of  segregation.  Or 
it  may  be  that  segregation  does  not  occur  owing  to  some 
definite  physiological  reason  which  at  present  we  do  not 
understand. 

And  here  it  is  impossible  not  to  recall  Mendel's  own 
experiences  with  the  Hawkweeds  (Hieracium) .  This 


xii  INTERMEDIATES  133 

genus  of  plants  exhibits  an  extraordinary  profusion  of 
forms  differing  from  one  another  sometimes  in  a  single 
feature,  sometimes  in  several.  The  question  as  to  how  far 
these  numerous  forms  were  to  be  classified  as  distinct 
species,  how  far  as  varieties,  and  how  far  as  products  of 
chance  hybridisation,  was  even  at  that  time  a  source  of 
keen  controversy  among  botanists.  There  is  little  doubt 
that  Mendel  undertook  his  experiments  on  the  Hawk- 
weeds  in  the  hope  that  the  conception  of  unit-characters 
so  brilliantly  demonstrated  for  the  pea  would  serve  to  ex- 
plain the  great  profusion  of  forms  among  the  Hieraciums. 
Owing  to  the  minute  size  of  their  florets,  these  plants  offer 
very  considerable  technical  difficulties  in  the  way  of  cross 
fertilisation.  By  dint  of  great  perseverance  and  labour, 
however,  Mendel  succeeded  in  obtaining  a  few  crosses 
between  different  forms.  These  hybrids  were  reared  and  a 
further  generation  produced  from  them,  and,  no  doubt 
somewhat  to  Mendel's  chagrin,  every  one  of  them  proved 
to  breed  true.  There  was  a  complete  absence  of  that  seg- 
regation of  characters  which  he  had  shown  to  exist  in 
peas  and  beans,  and  had  probably  looked  forward  with 
some  confidence  to  finding  in  Hieracium.  More  than 
thirty  years  passed  before  the  matter  was  cleared  up. 
To-day  we  know  that  the  peculiar  behaviour  of  the  hybrid 
Hieraciums  is  due  to  the  fact  that  they  normally  produce 
seed  by  a  peculiar  process  of  parthenogenesis.  It  is  pos- 
sible to  take  an  unopened  flower  and  to  shear  off  with  a 


134  MENDELISM  CHAP,  xn 

razor  all  the  male  organs  together  with  the  stigmata 
through  which  the  pollen  reaches  the  ovules.  The  flower, 
nevertheless,  sets  perfectly  good  seed.  But  the  cells 
from  which  the  seeds  develop  are  not  of  the  same  nature 
as  the  normal  ovules  of  a  plant.  They  are  not  gametes 
but  retain  the  double  structure  of  the  maternal  cells. 
They  are  rather  to  be  regarded  as  of  the  nature  of  buds 
which  early  become  detached  from  the  parent  stock  to 
lead  an  independent  existence,  and,  like  buds,  they  re- 
produce exactly  the  maternal  characteristics.  The  dis- 
covery of  the  true  nature  of  this  case  was  only  rendered 
possible  by  the  development  of  the  study  of  cytology,  and 
it  was  not  given  to  Mendel  to  live  long  enough  to  learn 
why  his  hybrid  Hieraciums  all  bred  true. 


CHAPTER  XIII 

VARIATION   AND   EVOLUTION 

THROUGH  the  facts  of  heredity  we  have  reached  a  new 
conception  of  the  individual.  Hitherto  we  have  been 
accustomed  to  distinguish  between  the  members  of  a 
family  of  rabbits  like  that  illustrated  on  Plate  I.  by  assign- 
ing to  each  an  individuality,  and  by  making  use  of  cer- 
tain external  features,  such  as  the  coat  colour  or  the  mark- 
ings, as  convenient  outward  signs  to  express  our  idea  that 
the  individuality  of  these  different  animals  is  different. 
Apart  from  this,  our  notions  as  to  what  constituted  the 
individuality  in  each  case  were  at  best  but  vague.  Men- 
delian  analysis  has  placed  in  our  hands  a  more  precise 
method  of  estimating  and  expressing  the  variations  that 
are  to  be  found  between  one  individual  and  another.  In- 
stead of  looking  at  the  individual  as  a  whole,  which  is  in 
some  vague  way  endowed  with  an  individuality  marking 
it  off  from  its  fellows,  we  now  regard  it  as  an  organism 
built  up  of  definite  characters  superimposed  on  a  basis  be- 
yond which  for  the  moment  our  analysis  will  not  take  us. 
We  have  begun  to  realise  that  each  individual  has  a  defi- 
nite architecture,  and  that  this  architecture  depends 

135 


136  MENDELISM  CHAP. 

primarily  upon  the  number  and  variety  of  the  factors  that 
existed  in  the  two  gametes  that  went  to  its  building. 
Now  most  species  exhibit  considerable  variation  and  exist 
in  a  number,  often  very  large,  of  more  or  less  well-defined 
varieties.  How  far  can  this  great  variety  be  explained  in 
terms  of  a  comparatively  small  number  of  factors  if  the 
number  of  possible  forms  depends  upon  the  number  of  the 
factors  which  may  be  present  or  absent  ? 
;  In  the  simple  case  where  the  homozygous  and  hetero- 
zygous conditions  are  indistinguishable  in  appearance 
the  number  of  possible  forms  is  2,  raised  to  the  power  of 
the  number  of  factors  concerned.  Thus  where  one  factor 
is  concerned  there  are  only  21  =  2  possible  forms,  where 
ten  factors  are  concerned  there  are  210  =  1024  possible 
forms  differing  from  one  another  in  at  most  ten  and  at 
least  one  character.  Where  the  factors  interact  upon  one 
another  this  number  will,  of  course,  be  considerably  in- 
creased. If  the  heterozygous  form  is  different  in  appear- 
ance from  the  homozygous  form,  there  are  three  possible 
forms  connected  with  each  factor ;  for  ten  such  factors  the 
possible  number  of  individuals  would  be  310  =  59,049  ;  for 
twenty  such  factors  the  possible  number  of  different  in- 
dividuals would  be  320  =  3,486,784,401.  The  presence 
or  absence  of  a  comparatively  small  number  of  factors  in 
a  species  carries  with  it  the  possibility  of  an  enormous 
range  of  individual  variation.  But  every  one  of  these  in- 
dividuals has  a  perfectly  definite  constitution  which  can 


xm  VARIATION  AND  EVOLUTION  137 

be  determined  in  each  case  by  the  ordinary  methods  of 
Mendelian  analysis.  For  in  every  instance  the  variation 
depends  upon  the  presence  or  absence  of  definite  factors 
carried  in  by  the  gametes  from  whose  union  the  individual 
results.  And  as  these  factors  separate  out  cleanly  in  the 
gametes  which  the  individual  forms,  such  variations  as 
depend  upon  them  are  transmitted  strictly  according  to 
the  Mendelian  scheme.  Provided  that  the  constitution 
of  the  gametes  is  unchanged,  the  heredity  of  such  varia- 
tion is  independent  of  any  change  in  the  conditions  of 
nutrition  or  environment  which  may  operate  upon  the 
individual  producing  the  gametes. 

But,  as  everybody  knows,  an  individual  organism, 
whether  plant  or  animal,  reacts,  and  often  reacts  mark- 
edly, to  the  environmental  conditions  under  which  its 
life  is  passed.  More  especially  is  this  to  be  seen  where 
such  characters  as  size  or  weight  are  concerned.  More 
sunlight  or  a  richer  soil  may  mean  stronger  growth  in  a 
plant,  better  nutrition  may  result  in  a  finer  animal,  su- 
perior education  may  lead  to  a  more  intelligent  man. 
But  although  the  changed  conditions  produce  a  direct 
effect  upon  the  individual,  we  have  no  indisputable  evi- 
dence that  such  alterations  are  connected  with  alterations 
in  the  nature  of  the  gametes  which  the  individual  pro- 
duces. And  without  this  such  variations  cannot  be 
perpetuated  through  heredity,  but  the  conditions  which 
produce  the  effect  must  always  be  renewed  in  each  succes- 


138  MENDELISM  CHAP. 

sive  generation.  We  are  led,  therefore,  to  the  conclusion 
that  two  sorts  of  variations  exist,  those  which  are  due  to 
the  presence  of  specific  factors  in  the  organism  and  those 
which  are  due  to  the  direct  effect  of  the  environment 
during  its  lifetime.  The  former  are  known  as  mutations, 
and  are  inherited  according  to  the  Mendelian  scheme; 
the  latter  have  been  termed  fluctuations,  and  at  present 
we  have  no  valid  reason  for  supposing  that  they  are  ever 
inherited.  For  though  instances  may  be  found  in  which 
effects  produced  during  the  lifetime  of  the  individual 
would  appear  to  affect  the  offspring,  this  is  not  necessarily 
due  to  heredity.  Thus  plants  which  are  poorly  nourished 
and  grown  under  adverse  conditions  may  set  seed  from 
which  come  plants  that  are  smaller  than  the  normal  al- 
though grown  under  most  favorable  conditions.  It  is 
natural  to  attribute  the  smaller  size  of  the  offspring  to  the 
conditions  under  which  the  parents  were  grown,  and 
there  is  no  doubt  that  we  should  be  quite  right  in  doing 
so.  Nevertheless,  it  need  have  nothing  to  do  with  hered- 
ity. As  we  have  already  pointed  out,  the  seed  is  a  larval 
plant  which  draws  its  nourishment  from  the  mother. 
The  size  of  the  offspring  is  affected  because  the  poorly 
nourished  parent  offered  a  bad  environment  to  the  young 
plant,  and  not  because  the  gametes  of  the  parent  were 
changed  through  the  adverse  conditions  under  which  it 
grew.  The  parent  in  this  case  is  not  only  the  producer  of 
gametes,  but  also  a  part  of  the  environment  of  the  young 


xin  VARIATION  AND  EVOLUTION  139 

plant,  and  it  is  in  this  latter  capacity  that  it  affects  its  off- 
spring. Wherever,  as  in  plants  and  mammals,  the  organ- 
ism is  parasitic  upon  the  mother  during  its  earlier  stages, 
,the  state  of  nutrition  of  the  latter  will  almost  certainly 
react  upon  it,  and  in  this  way  a  semblance  of  transmitted 
weakness  or  vigour  is  brought  about.  Such  a  connection 
between  mother  and  offspring  is  purely  one  of  environ- 
ment, and  it  cannot  be  too  strongly  emphasised  that  it 
has  nothing  to  do  with  the  ordinary  process  of  heredity. 
The  distinction  between  these  two  kinds  of  varia- 
tion, so  entirely  different  in  their  causation,  renders 
it  possible  to  obtain  a  clearer  view  of  the  process  of  evo- 
lution than  that  recently  prevalent.  As  Darwin  long 
ago  realised,  any  theory  of  evolution  must  be  based  upon 
the  facts  of  heredity  and  variation.  Evolution  only 
comes  about  through  the  survival  of  certain  variations 
and  the  elimination  of  others.  But  to  be  of  any  moment 
in  evolutionary  change  a  variation  must  be  inherited. 
And  to  be  inherited  it  must  be  represented  in  the  gametes. 
This,  as  we  have  seen,  is  the  case  for  those  variations 
which  we  have  termed  mutations.  For  the  inheritance 
of  fluctuations,  on  the  other  hand,  of  the  variations 
which  result  from  the  direct  action  of  the  environment 
upon  the  individual,  there  is  no  indisputable  evidence. 
Consequently  we  have  no  reason  for  regarding  them  as 
playing  any  part  in  the  production  of  that  succession  of 
temporarily  stable  forms  which  we  term  evolution.  In 


140  MENDELISM  CHAP. 

the  light  of  our  present  knowledge  we  must  regard  the 
mutation  as  the  basis  of  evolution  —  as  the  material  upon 
which  natural  selection  works.  For  it  is  the  only  form 
of  variation  of  whose  heredity  we  have  any  certain 
knowledge. 

It  is  evident  that  this  view  of  the  process  of  evolution 
is  in  some  respects  at  variance  with  that  generally  held 
during  the  past  half  century.  There  we  were  given  the 
conception  of  an  abstract  type  representing  the  species, 
and  from  it  most  of  the  individuals  diverged  in  various 
directions,  though,  generally  speaking,  only  to  a  very 
small  extent.  It  was  assumed  that  any  variation,  how- 
ever small,  might  have  a  selection  value,  that  is  to  say, 
could  be  transmitted  to  the  offspring.  Some  of  these 
would  possess  it  in  a  less  and  some  in  a  greater  degree 
than  the  parent.  If  the  variation  were  a  -useful  one, 
those  possessing  to  a  rather  greater  extent  would  be 
favoured  through  the  action  of  natural  selection  at  the 
expense  of  their  less  fortunate  brethren,  and  would  leave 
a  greater  number  of  offspring,  of  whom  some  possessed  it 
in  an  even  more  marked  degree  than  themselves.  And 
so  it  would  go  on.  The  process  was  a  cumulative  one. 
The  slightest  variation  in  a  favourable  direction  gave 
natural  selection  a  starting-point  to  work  on.  Through 
the  continued  action  of  natural  selection  on  each  succes- 
sive generation  the  useful  variation  was  gradually  worked 
up,  until  at  last  it  reached  the  magnitude  of  a  specific  dis- 


xm  VARIATION  AND  EVOLUTION  141 

tinction.  Were  it  possible  in  such  a  case  to  have  all  the 
forms  before  us,  they  would  present  the  appearance  of  a 
long  series  imperceptibly  grading  from  one  extreme  to  the 
other. 

Upon  this  view  are  made  two  assumptions  not  unnatu- 
ral in  the  absence  of  any  exact  knowledge  of  the  nature  of 
heredity  and  variation.  It  was  assumed,  in  the  first  place 
that  variation  was  a  continuous  process,  and,  second,  that 
any  variation  could  be  transmitted  to  the  offspring. 
Both  of  these  assumptions  have  since  been  shown  to  be 
unjustified.  Even  before  Mendel's  work  became  known 
Bateson  had  begun  to  call  attention  to  the  prevalence  of 
discontinuity  in  variation,  and  a  few  years  later  this  was 
emphasised  by  the  Dutch  botanist  Hugo  de  Vries  in  his 
great  work  on  The  Mutation  Theory.  The  ferment  of 
new  ideas  was  already  working  in  the  solution,  and  under 
the  stimulus  of  Mendel's  work  they  have  rapidly  crys- 
tallised out.  With  the  advent  of  heredity  as  a  definite 
science  we  have  been  led  to  revise  our  views  as  to  the 
nature  of  variation,  and  consequently  in  some  respects  as 
to  the  trend  of  evolution.  Heritable  variation  has  a 
definite  basis  in  the  gamete,  and  it  is  to  the  gamete, 
therefore,  not  to  the  individual,  that  we  must  look  for 
the  initiation  of  this  process.  Somewhere  or  other  in  the 
course  of  their  production  is  added  or  removed  the  factor 
upon  whose  removal  or  addition  the  new  variation  owes 
its  existence.  The  new  variation  springs  into  being  by  a 


142  MENDELISM  CHAP. 

sudden  step,  not  by  a  process  of  gradual  and  almost 
imperceptible  augmentation.  It  is  not  continuous  but 
discontinuous,  because  it  is  based  upon  the  presence  or 
absence  of  some  definite  factor  or  factors  —  upon  discon- 
tinuity in  the  gametes  from  which  it  sprang.  Once 
formed,  its  continued  existence  is  subject  to  the  arbitra- 
ment of  natural  selection.  If  of  value  in  the  struggle  for 
existence  natural  selection  will  decide  that  those  who 
possess  it  shall  have  a  better  chance  of  survival  and  of 
leaving  offspring  than  those  who  do  not  possess  it.  If  it 
is  harmful  to  the  individual  natural  selection  will  soon 
bring  about  its  elimination.  But  if  the  new  variation  is 
neither  harmful  nor  useful  there  seems  no  reason  why  it 
should  not  persist. 

In  this  way  we  avoid  a  difficulty  that  beset  the  older 
view..  For  on  that  view  no  new  character  could  be  de- 
veloped except  by  the  piling  up  of  minute  variations 
through  the  action  of  natural  selection.  Consequently 
any  character  found  in  animals  and  plants^  must  be  sup- 
posed to  be  of  some  definite  use  to  the  individual.  Other- 
wise it  could  not  have  developed  through  the  action  of 
natural  selection.  But  there  are  plenty  of  characters 
to  which  it  is  exceedingly  difficult  to  ascribe  any  utility, 
and  the  ingenuity  of  the  supporters  of  this  view  has  often 
been  severely  taxed  to  account  for  their  existence.  On 
the  more  modern  view  this  difficulty  is  avoided.  The 
origin  of  a  new  variation  is  independent  of  natural  selec- 


xiii  VARIATION  AND  EVOLUTION  143 

tion,  and  provided  that  it  is  not  directly  harmful  there  is 
no  reason  why  it  should  not  persist.  In  this  way  we  are 
released  from  the  burden  of  discovering  a  utilitarian 
motive  behind  all  the  multitudinous  characters  of  living 
.organisms.  For  we  now  recognise  that  the  function  of 
natural  selection  is  selection  and  not  creation.  It  has 
nothing  to  do  with  the  formation  of  the  new  variation. 
It  merely  decides  whether  it  is  to  survive  or  to  be 
eliminated. 

One  of  the  arguments  made  use  of  by  supporters  of  the 
older  view  is  that  drawn  from  the  study  of  adaptation. 
Animals  and  plants  are  as  a  rule  remarkably  well  adapted 
to  living  the  life  which  their  surroundings  impose  upon 
them,  and  in  some  cases  this  adaptation  is  exceedingly 
striking.  Especially  is  this  so  in  the  many  instances  of 
what  is  called  protective  coloration,  where  the  animal 
comes  to  resemble  its  surroundings '  so  closely  that  it 
may  reasonably  be  supposed  to  cheat  even  the  keenest 
sighted  enemy.  Surely,  we  are  told,  such  perfect  adap- 
tation could  hardly  have  arisen  through  the  mere  sur- 
vival of  chance  sports.  Surely  there  must  be  some  guid- 
ing hand  moulding  the  species  into  the  required  shape. 
The  argument  is  an  old  one.  For  John  Ray  that  guiding 
hand  was  the  superior  wisdom  of  the  Creator :  for  the 
modern  Darwinian  it  is  Natural  Selection  controlling  the 
direction  of  variation.  Mendelism  certainly  offers  no 
suggestion  of  any  such  controlling  force.  It  interprets  the 


144  MENDELISM  CHAP. 

variations  of  living  forms  in  terms  of  definite  physiological 
factors,  and  the  diversity  of  animal  and  plant  life  is  due 
to  the  gain  or  loss  of  these  factors,  to  the  origination  of 
new  ones,  or  to  fresh  combinations  among  those  already  in 
existence.  Nor  is  there  any  valid  reason  against  the  sup- 
position that  even  the  most  remarkable  cases  of  resem- 
blance, such  as  that  of  the  leaf  insect,  may  have  arisen 
through  a  process  of  mutation.  Experience  with  domestic 
plants  and  animals  shows  that  the  most  bizarre  forms  may 
arise  as  sports  and  perpetuate  themselves.  Were  such 
forms,  arising  under  natural  conditions,  to  be  favoured 
by  natural  selection  owing  to  a  resemblance  to  something 
in  their  environment  we  should  obtain  a  striking  case 
of  protective  adaptation.  And  here  it  must  not  be  for- 
gotten that  those  striking  cases  to  which  our  attention 
is  generally  called  are  but  a  very  small  minority  of  the 
existing  forms  of  life. 

For  that  special  group  of  adaptation  phenomena  classed 
under  the  head  of  Mimicry,  Mendelism  seems  to  offer 
an  interpretation  simpler  than  that  at  present  in  vogue. 
This  perhaps  may  be  more  clearly  expressed  by  taking  a 
specific  case.  There  is  in  Africa  a  genus  of  Danaine 
butterflies  known  as  Amauris,  and  there  are  reasons  for 
considering  that  the  group  to  which  it  belongs  possesses 
properties  which  render  it  unpalatable  to. vertebrate  ene- 
mies such  as  birds  or  monkeys.  In  the  same  region  is 
also  found  the  genus  Euralia  belonging  to  the  entirely 


xin  VARIATION  AND  EVOLUTION  145 

different  family  of  the  Nymphalidae,  to  which  there  is  no 
evidence  for  assigning  the  disagreeable  properties  of  the 
Danaines.  Now  the  different  species  of  Euralia  show  re- 
markably close  resemblances  to  the  species  of  Amauris, 
which  are  found  flying  in  the  same  region,  and  it  is  sup- 
posed that  by  " mimicking"  the  unpalatable  forms  they 
impose  upon  their  enemies  and  thereby  acquire  immunity 
from  attack.  The  point  at  issue  is  the  way  in  which  this 
seemingly  purposeful  resemblance  has  been  brought 
about. 

One  of  the  species  of  Euralia  occurs  in  two  very  dis- 
tinct forms  (PI.  VI.),  which  were  previously  regarded  as 
separate  species  under  the  names  E.  wahlbergi  and  E. 
mima.  These  two  forms  respectively  resemble  Amauris 
dominicanus  and  A.  echeria.  For  purposes  of  argument 
we  will  assume  A.  echeria  to  be  the  more  recent  form  of 
the  two.  On  the  modern  Darwinian  view  certain  in- 
dividuals of  A .  dominicanus  gradually  diverged  from  the 
dominicanus  type  and  eventually  reached  the  echeria  type, 
though  why  this  should  have  happened  does  not  appear 
to  be  clear.  At  the  same  time  those  specimens  which 
tended  to  vary  in  the  direction  of  A.  echeria  in  places 
where  this  species  was  more  abundant  than  A .  dominicanus 
were  encouraged  by  natural  selection,  and  under  its 
guiding  hand  the  form  mima  eventually  arose  from  wahl- 
bergi. 

According   to   Mendelian  views,  on  the   other  hand, 


146  MENDELISM  CHAP. 

A.  echeria  arose  suddenly  from  A.  dominicanus  (or  vice 
versa),  and  similarly  mima  arose  suddenly  from  wahlbergi. 
If  mima  occurred  where  A.  echeria  was  common  and  A. 
dominicanus  was  rare,  its  resemblance  to  the  more  plenti- 
ful distasteful  form  would  give  it  the  advantage  over  wahl- 
bergi and  allow  it  to  establish  itself  in  place  of  the  latter. 
On  the  modern  Darwinian  view  natural  selection  gradually 
shapes  wahlbergi  into  the  mima  form  owing  to  the  pres- 
ence of  A .  echeria ;  on  the  Mendelian  view  natural  selec- 
tion merely  conserves  the. mima  form  when  once  it  has 
arisen.  Now  this  case  of  mimicry  is  one  of  especial  in- 
terest, because  we  have  experimental  evidence  that  the 
relation  between  mima  and  wahlbergi  is  a  simple  Men- 
delian one,  though  at  present  it  is  uncertain  which  is  the 
dominant  and  which  the  recessive  form.  The  two  have 
been  proved  to  occur  in  families  bred  from  the  same 
female  without  the  occurrence  of  any  intermediates,  and 
the  fact  that  the  two  segregate  cleanly  is  strong  evidence 
in  favour  of  the  Mendelian  view.  On  this  view  the  gen- 
era Amauris  and  Euralia  contain  a  similar  set  of  pattern 
factors,  and  the  conditions,  whatever  they  may  be, 
which  bring  about  mutation  in  the  former  lead  to  the 
production  of  a  similar  mutation  in  the  latter.  Of  the 
different  forms  of  Euralia  produced  in  any  region  that 
one  has  the  best  chance  of  survival,  through  the  opera- 
tion of  natural  selection,  which  resembles  the  most  plen- 
tiful Amauris  form.  Mimetic  resemblance  is  a  true  phe- 


PLATE  VI. 


xiii  VARIATION  AND  EVOLUTION  147 

nomenon,  but  natural  selection  plays  the  part  of  a  con- 
servative, not  of  a  formative  agent. 

It  is  interesting  to  recall  that  in  earlier  years  Darwin 
was  inclined  to  ascribe  more  importance  to  " sports"  as 
opposed  to  continuous  minute  variation,  and  to  consider 
that  they  might  play  a  not  inconsiderable  part  in  the 
formation  of  new  varieties  in  nature.  This  view,  how- 
ever, he  gave  up  later,  because  he  thought  that  the  rela- 
tively rare  sport  or  mutation  would  rapidly  disappear 
through  the  swamping  effects  of  crossing  with  the  more 
abundant  normal  form,  and  so,  even  though  favoured  by 
natural  selection,  would  never  succeed  in  establishing 
itself.  Mendel's  discovery  has  eliminated  this  diffi- 
culty. For  suppose  that  the  sport  differed  from  the 
normal  in  the  loss  of  a  factor  and  were  recessive.  When 
mated  with  the  normal  this  character  would  seem  to  dis- 
appear, though,  of  course,  half  of  the  gametes  of  its  prog- 
eny would  bear  it.  By  continual  crossing  with  normals 
a  small  proportion  of  heterozygotes  would  eventually  be 
scattered  among  the  population,  and  as  soon  as  any  two 
of  these  mated  together  the  recessive  sport  would  ap- 
pear in  one  quarter  of  their  offspring. 

A  suggestive  contribution  to  this  subject  was  recently 
made  by  G.  H.  Hardy.  Considering  the  distribution  of 
a  single  factor  in  a  mixed  population  consisting  of  the  het- 
erozygous and  the  two  homozygous  forms  he  showed  that 
such  a  population  breeding  at  random  rapidly  fell  into  a 


148  MENDELISM  CHAP. 

stable  condition  with  regard  to  the  proportion  of  these 
three  forms,  whatever  may  have  been  the  proportion 
of  the  three  forms  to  start  with.  Let  us  suppose,  for 
instance,  that  the  population  consists  of  p  homozygotes 
of  one  kind,  r  homozygotes  of  the  other  kind,  and  2  q 
heterozygotes.  Hardy  pointed  out  that,  other  things 
being  equal,  such  a  population  would  be  in  equilibrium 
for  this  particular  factor  so  long  as  the  condition  (f  =  pr 
was  fulfilled.  If  the  condition  is  fulfilled  to  start  with,  the 
population  remains  in  equilibrium.  If  the  condition  is 
not  fulfilled  to  start  with,  Hardy  showed  that  a  position 
of  equilibrium  becomes  established  after  a  single  gener- 
ation, and  that  this  position  is  thereafter  maintained. 
The  proportions  of  the  three  classes  which  satisfy  the 
equation  q~  =  pr  are  exceedingly  numerous,  and  popula- 
tions in  which  they  existed  in  the  proportions  shown  in 
the  appended  table  would  remain  in  stable  equilibrium 
generation  after  generation :  — 

P  2q.  r. 

12  I 

i  4  4 

i  6  9 

i  8  16 

I  20,000  IOO,OCO,QOO 

i  2  n  ri* 

This,  of  course,  assumes  that  all  three  classes  are  equally 
fertile,  and  that  no  form  of  selection  is  taking  place  to  the 


xm  VARIATION  AND  EVOLUTION  149 

benefit  of  one  class  more  than  of  another.  Moreover,  it 
makes  no  difference  whether  p  represents  the  homo- 
zygous  dominants  or  whether  it  stands  for  the  recessives. 
A  population  containing  a  very  small  proportion  of  domi- 
nants and  one  containing  a  similar  proportion  of  recessives 
are  equally  stable.  The  term  dominant  is  in  some  re- 
spects apt  to  be  misleading,  for  a  dominant  character 
cannot  in  virtue  of  its  dominance  establish  itself  at  the 
expense  of  a  recessive  one.  Brown  eyes  in  man  are 
dominant  to  blue,  but  there  is  no  reason  to  suppose  that 
as  years  go  on  the  population  of  these  islands  will  become 
increasingly  brown  eyed.  Given  equality  of  conditions 
both  are  on  an  equal  footing.  If,  however,  either  domi- 
nant or  recessive  be  favoured  by  selection  the  conditions 
are  altered,  and  it  can  be  shown  that  even  a  small  advan- 
tage possessed  by  the  one  will  rapidly  lead  to  the  elimina- 
tion of  the  other.  Even  with  but  a  5  per  cent  selection 
advantage  in  its  favour  it  can  be  shown  that  a  rare  sport 
will  oust  the  normal  form  in  a  few  hundred  generations. 
In  this  way  we  are  freed  from  a  difficulty  inherent  in  the 
older  view  that  varieties  arose  through  a  long-continued 
process  involving  the  accumulation  of  very  slight  varia- 
tions. On  that  view  the  establishing  of  a  new  type  was 
of  necessity  a  very  long  and  tedious  business,  involving 
many  thousands  of  generations.  For  this  reason  the  biol- 
ogist has  been  accustomed  to  demand  a  very  large  sup- 
ply of  time,  often  a  great  deal  more  than  the  physicist  is 


150  MENDELISM  CHAP. 

disposed  to  grant,  and  this  has  sometimes  led  him  to 
expostulate  with  the  latter  for  cutting  off  the  supply. 
On  the  newer  views,  however,  this  difficulty  need  not 
arise,  for  we  realise  that  the  origin  and  establishing  of  a 
new  form  may  be  a  very  much  more  rapid  process  than 
has  hitherto  been  deemed  possible. 

One  last  question  with  regard  to  evolution.  How  far 
does  Mendelism  help  us  in  connection  with  the  problem 
of  the  origin  of  species  ?  Among  the  plants  and  animals 
with  which  we  have  dealt  we  have  been  able  to  show 
that  distinct  differences,  often  considerable,  in  colour, 
size,  and  structure,  may  be  interpreted  in  terms  of  Men- 
delian  factors.  It  is  not  unlikely  that  most  of  the  vari- 
ous characters  which  the  systematist  uses  to  mark  off 
one  species  from  another,  the  so-called  specific  characters, 
are  of  this  nature.  They  serve  as  convenient  labels,  but 
are  not  essential  to  the  conception  of  species.  A  sys- 
tematist who  denned  the  wild  sweet  pea  could  hardly  fail 
to  include  in  his  definition  such  characters 'as  the  pro- 
cumbent habit,  the  tendrils,  the  form  of  the  pollen,  the 
shape  of  the  flower,  and  its  purple  colour.  Yet  all  these 
and  other  characters  have  been  proved  to  depend  upon 
the  presence  of  definite  factors  which  can  be  removed  by 
appropriate  crossing.  By  this  means  we  can  produce  a 
small  plant  a  few  inches  in  height  with  an  erect  habit  of 
growth,  without  tendrils,  with  round  instead  of  oblong  pol- 
len, and  with  colourless  deformed  flowers  quite  different 


xm  VARIATION  AND  EVOLUTION  151 

in  appearance  from  those  of  the  wild  form.  Such  a  plant 
would  breed  perfectly  true,  and  a  botanist  to  whom  it 
was  presented,  if  ignorant  of  its  origin,  might  easily  rele- 
gate it  to  a  different  genus.  Nevertheless,  though  so 
widely  divergent  in  structure,  such  a  plant  must  yet  be 
regarded  as  belonging  to  the  species  Lathyrus  odoratus. 
For  it  still  remains  fertile  with  the  many  different  varie- 
ties of  sweet  pea.  It  is  not  visible  attributes  that  con- 
stitute the  essential  difference  between  one  species  and 
another.  The  essential  difference,  whatever  it  may  be,  is 
that  underlying  the  phenomenon  of  sterility.  The  visi- 
ble attributes  are  those  made  use  of  by  the  systematist  in 
cataloguing  the  different  forms  of  animal  and  plant  life,  for 
he  has  no  other  choice.  But  it  must  not  be  forgotten  that 
they  are  often  misleading.  Until  they  were  bred  together 
Euralia  wahlbergi  and  E.  mima  were  regarded  as  perfectly 
valid  species,  and  there  is  little  doubt  that  numbers  of 
recognised  species  will  eventually  fall  to  the  ground  in  the 
same  way  as  soon  as  we  are  in  a  position  to  apply  «the 
test  of  breeding.  Mendelism  has  helped  us  to  realise  that 
specific  characters  may  be  but  incidental  to  a  species  — 
that  the  true  criterion  of  what  constitutes  a  species  is 
sterility,  and  that  particular  form  of  sterility  which  pre- 
vents two  healthy  gametes  on  uniting  from  producing  a 
zygote  with  normal  powers  of  growth  and  reproduction. 
For  there  are  forms  of  sterility  which  are  purely  mechan- 
ical. The  pollen  of  Mirabilis  jalapa  cannot  fertilise  M . 


152  MENDELISM  CHAP,  xm 

longiflora,  because  the  pollen  tubes  of  the  former  are  not 
long  enough  to  penetrate  down  to  the  ovules  of  the  latter. 
Hybrids  can  nevertheless  be  obtained  from  the  reciprocal 
cross.  Nor  should  we  expect  offspring  from  a  St.  Ber- 
nard and  a  toy  terrier  without  recourse  to  artificial 
fertilisation.  Or  sterility  may  be  due  to  pathological 
causes  which  prevent  the  gametes  from  meeting  one  an- 
other in  a  healthy  state.  But  in  most  cases  it  is  prob- 
able that  the  sterility  is  due  to  some  other  cause.  It  is 
not  inconceivable  that  definite  differences  in  chemical 
composition  render  the  protoplasm  of  one  species  toxic 
to  the  gametes  of  the  other,  and  if  this  is  so  it  is  not  im- 
possible that  we  may  some  day  be  able  to  express  these 
differences  in  terms  of  Mendelian  factors.  The  very 
nature  of  the  case  makes  it  one  of  extreme  difficulty  for 
experimental  investigation.  At  any  rate,  we  realise 
more  clearly  than  before  that  the  problem  of  species 
is  not  one  that  can  be  resolved  by  the  study  of  mor- 
phology or  of  sy  sterna  tics.  It  is  a  problem  in  physi- 
ology. 


CHAPTER  XIV 

ECONOMICAL 

SINCE  heredity  lies  at  the  basis  of  the  breeder's  work, 
it  is  evident  that  any  contribution  to  a  more  exact  know- 
ledge of  this  subject  must  prove  of  service  to  him,  and 
there  is  no  doubt  that  he  will  be  able  to  profit  by  Men- 
delian  knowledge  in  the  conduct  of  his  operations.  In- 
deed, as  we  shall  see  later,  these  ideas  have  already  led 
to  striking  results  in  the  raising  of  new  and  more  profitable 
varieties.  In  the  first  place,  heredity  is  a  question  of  in- 
dividuals. Identity  of  appearance  is  no  sure  guide  to 
reproductive  qualities.  Two  individuals  similarly  bred 
and  indistinguishable  in  outward  form  may  nevertheless 
behave  entirely  differently  when  bred  from.  Take,  for 
instance,  the  family  of  sweet  peas  shown  on  Plate  IV. 
The  F2  generation  here  consists  of  seven  distinct  types, 
three  sorts  of  purples,  three  sorts  of  reds,  and  whites. 
Let  us  suppose  that  our  object  is  to  obtain  a  true  breeding 
strain  of  the  pale  purple  picotee  form.  Now  from  the 
proportions  in  which  they  come  we  know  that  the  dilute 
colour  is  due  to  the  absence  of  the  factor  which  intensifies 
the  colour.  Consequently  the  picotee  cannot  throw  the 

153 


154  MENDELISM  CHAP. 

two  deeper  shades  of  red  or  purple.  But  it  may  be  hetero- 
zygous for  the  purpling  factor  when  it  will  throw  the  dilute 
red  (tinged  white),  or  it  may  be  heterozygous  for  either  or 
both  of  the  two  colour  factors  (cf.  p.  44),  in  which  case  it 
will  throw  whites.  Of  the  picotees  which  come  in  such 
a  family,  therefore,  some  will  give  picotees,  tinged  whites, 
and  whites,  others  will  give  picotees  and  tinged  whites 
only,  others  will  give  picotees  and  whites  only,  while 
others,  again,  and  these  the  least  numerous,  will  give 
nothing  but  picotees.  The  new  variety  is  already  fixed 
in  a  certain  definite  proportion  of  the  plants;  in  this 
particular  instance  in  i  out  of  every  27.  All  that  re- 
mains to  be  done  is  to  pick  out  these  plants.  Since  all 
the  picotees  look  alike,  whatever  their  breeding  capacity, 
the  only  way  to  do  this  is  to  save  the  seed  from  a  number 
of  such  plants  individually,  and  to  raise  a  further  genera- 
tion. Some  of  them  will  be  found  to  breed  true.  The 
variety  is  then  established,  and  may  at  once  be  put  on 
the  market  with  full  confidence  that  it  will  hereafter 
throw  none  of  the  other  forms.  The  all-important  thing 
is  to  save  and  sow  the  seed  of  separate  individuals  sepa- 
rately. However  alike  they  look,  the  seed  from  differ- 
ent individuals  must  on  no  account  be  mixed.  Provided 
that  due  care  is  taken  in  this  respect  no  long  and  tedious 
process  of  selection  is  required  for  the  fixation  of  any 
given  variety.  Every  possible  variety  arising  from  a 
cross  appears  in  the  F2  generation  if  only  a  sufficient  num- 


xiv  ECONOMICAL  155 

her  is  raised,  and  of  all  these  different  varieties  a  certain 
proportion  of  each  is  already  fixed.  Heredity  is  a  ques- 
tion of  individuals,  and  the  recognition  of  this  will  save 
the  breeder  much  labour,  and  enable  him  to  fix  his  varie- 
'ties  in  the  shortest  possible  time. 

Such  cases  as  these  of  the  sweet  pea  throw  a  fresh  light 
upon  another  of  the  breeder's  conceptions,  that  of  purity 
of  type.  Hitherto  the  criterion  of  a  " pure-bred"  thing, 
whether  plant  or  animal,  has  been  its  pedigree,  and  the  in- 
dividual was  regarded  as  more  or  less  pure  bred  for  a  given 
quality  according  as  it  could  show  a  longer  or  shorter  list 
of  ancestors  possessing  this  quality.  To-day  we  realise 
that  this  is  not  essential.  The  pure-bred  picotee  appears 
in  our  ¥2  family  though  its  parent  was  a  purple  bicolor, 
and  its  remoter  ancestors  whites  for  generations.  So  also 
from  the  cross  between  pure  strains  of  black  and  albino 
rabbits  we  may  obtain  in  the  F2  generation  animals  of  the 
wild  agouti  colour  which  breed  as  true  to  type  as  the  pure 
wild  rabbit  of  irreproachable  pedigree.  The  true  test  of 
the  pure  breeding  thing  lies  not  in  its  ancestry  but  in  the 
nature  of  the  gametes  which  have  gone  to  its  making. 
Whenever  two  similarly  constituted  gametes  unite,  what- 
ever the  nature  of  the  parents  from  which  they  arose,  the 
resulting  individual  is  homozygous  in  all  respects  and 
must  consequently  breed  true.  In  deciding  questions  of 
purity  it  is  to  the  gamete,  and  not  to  ancestry,  that  out 
appeal  must  henceforth  be  made. 


156  MENDELISM  CHAP. 

Improvement  is  after  all  the  keynote  to  the  breeder's 
operations.  He  is  aiming  at  the  production  of  a  strain 
which  shall  combine  the  greatest  number  of  desirable 
properties  with  the  least  number  of  undesirable  ones. 
This  good  quality  he  must  take  from  one  strain,  that  from 
another,  and  that  again  from  a  third,  while  at  the  same 
time  avoiding  all  the  poor  qualities  that  these  different 
strains  possess.  It  is  evident  that  the  Mendelian  con- 
ception of  characters  based  upon  definite  factors  which 
are  transmitted  on  a  definite  scheme  must  prove  of  the 
greatest  service  to  him.  For  once  these  factors  have  been 
determined,  their  distribution  is  brought  under  control, , 
and  they  can  be  associated  together  or  dissociated  at  the 
breeder's  will.  The  chief  labour  involved  is  that  neces- 
sary for  the  determination  of  the  factors  upon  which  the 
various  characters  depend.  For  it  often  happens  that 
what  appears  to  be  a  simple  character  turns  out  when 
analysed  to  depend  upon  the  simultaneous  presence  of 
several  distinct  factors.  Thus  the  Malay  fowl  breeds 
true  to  the  walnut  comb,  as  does  also  the  Leghorn  to  the 
single  comb,  and  when  pure  strains  are  crossed  all  the 
offspring  have  walnut  combs.  At  first  sight  it  would  be 
not  unnatural  to  regard  the  difference  as  dependent  upon 
the  presence  or  absence  of  a  single  factor.  Yet,  as  we 
have  already  seen,  two  other  types  of  comb,  the  pea  and 
the  rose,  make  their  appearance  in  the  F2  generation. 
Analysis  shows  that  the  difference  between  the  walnut 


xiv  ECONOMICAL  157 

and  the  single  is  a  difference  of  two  factors,  and  it  is  not 
until  this  has  been  determined  that  we  can  proceed  with 
certainty  to  transfer  the  walnut  character  to  a  single- 
combed  breed.  Moreover,  in  his  process  of  analysis  the 
breeder  must  be  prepared  to  encounter  the  various  phe- 
nomena that  we  have  described  under  the  headings  of 
interaction  of  factors,  coupling  and  repulsion,  and  the 
recognition  of  these  phenomena  will  naturally  influence 
his  procedure.  Or  again,  his  experiments  may  show  him 
that  one  of  the  characters  he  wants,  like  the  blue  of  the 
Andalusian  fowl,  is  dependent  upon  the  heterozygous 
nature  of  the  individual  which  exhibits  it,  and  if  such  is 
the  case  he  will  be  wise  to  refrain  from  any  futile  attempt 
at  fixing  it.  If  it  is  essential  it  must  be  built  up  again  in 
each  generation,  and  he  will  recognise  that  the  most  ec- 
onomical way  of  doing  this  is  to  cross  the  two  pure  strains 
so  that  all  the  offspring  may  possess  the  desired  charac- 
ter. The  labour  of  analysis  is  often  an  intricate  and  te- 
dious business.  But  once  done  it  is  done  once  for  all.  As 
soon  as  the  various  factors  are  determined,  upon  which 
the  various  characters  of  the  individual  depend,  as  soon 
as  the  material  to  be  made  use  of  has  been  properly  ana- 
lysed, the  production  and  fixation  of  the  required  combi- 
nations becomes  a  matter  of  simple  detail. 

An  excellent  example  of  the  practical  application  of 
Mendelian  principles  is  afforded  by  the  experiments  which 
Professor  Biffen  has  recently  carried  out  in  Cambridge. 


i$8  MENDELISM  CHAP. 

Taken  as  a  whole  English  wheats  compare  favourably 
with  foreign  ones  in  respect  of  their  cropping  power.  On 
the  other  hand,  they  have  two  serious  defects.  They  are 
liable  to  suffer  from  the  attacks  of  the  fungus  which  causes 
rust,  and  they  do  not  bake  into  a  good  loaf.  This  last 
property  depends  upon  the  amount  of  gluten  present,  and 
it  is  the  greater  proportion  of  this  which  gives  to  the 
"hard"  foreign  wheat  its  quality  of  causing  the  loaf  to 
rise  well  when  baked.  For  some  time  it  was  held  that 
"hard"  wheat  with  a  high  glutinous  content  could  not 
be  grown  in  the  English  climate,  and  undoubtedly  most  of 
the  hard  varieties  imported  for  trial  deteriorated  greatly 
in  a  very  short  time.  Professor  Biff  en  managed  to  obtain 
a  hard  wheat  which  kept  its  qualities  when  grown  in 
England.  But  in  spite  of  the  superior  quality  of  its 
grain  from  the  baker's  point  of  view  its  cropping  capacity 
was  too  low  for  it  to  be  grown  profitably  in  competition 
with  English  wheats.  Like  the  latter,  it  was  also  subject 
to  rust.  Among  the  many  varieties  which  Professor  Bif- 
fen  collected  and  grew  for  observation  he  managed  to  find 
one  which  was  completely  immune  to  the  attacks  of  the 
rust  fungus,  though  in  other  respects  it  had  no  desirable 
quality  to  recommend  it.  Now  as  the  result  of  an  elab- 
orate series  of  investigations  he  was  able  to  show  that  the 
qualities  of  heavy  cropping  capacity,  "hardness"  of  grain, 
and  immunity  to  rust  can  all  be  expressed  in  terms  of 
Mendelian  factors.  Having  once  analysed  his  material 


XIV 


ECONOMICAL 


159 


the  rest  was  comparatively  simple,  and  in  a  few  years  he 
has  been  able  to  build  up  a  strain  of  wheat  which  com- 
bines the  cropping  capacity  of  the  best  English  varieties 
with  the  hardness  of  the  foreign  kinds,  and  at  the  same 
time  is  completely  immune  to  rust.  This  wheat  has  al- 
ready been  shown  to  keep  its  qualities  unchanged  for  sev- 
eral years,  and  there  is  little  doubt  that  when  it  comes  to 
be  grown  in  quantity  it  will  exert  an  appreciable  in- 
fluence on  wheat-growing  in  Great  Britain. 

It  may  be  objected  that  it  is  often  with  small  differ- 
ences rather  than  with  the  larger  and  more  striking  ones 


300 


200 


4       6       8      10     12     14      16     18     20 

"Weight  of  individual  seeda 

FIG.  30. 
Curves  to  illustrate  the  influence  of  selection. 

that  the  breeder  is  mainly  concerned.  It  does  not  matter 
much  to  him  whether  the  colour  of  a  pea  flower  is  purple 
or  pink  or  white.  But  it  does  matter  whether  the  plant 
bears  rather  larger  seeds  than  usual,  or  rather  more  of 
them.  Even  a  small  difference  when  multiplied  by  the 


160  MENDELISM  CHAP. 

size  of  the  crop  will  effect  a  considerable  difference  in  the 
profit.  It  is  the  general  experience  of  seedsmen  and 
others  that  differences  of  this  nature  are  often  capable 
of  being  developed  up  to  a  certain  point  by  a  process  of 
careful  selection  each  generation.  At  first  sight  this  ap- 
pears to  be  something  very  like  the  gradual  accumulation 
of  minute  variations  through  the  continuous  application 
of  a  selective  process.  Some  recent  experiments  by  Pro- 
fessor Johannsen  of  Copenhagen  set  the  matter  in  a  differ- 
ent light.  One  of  his  investigations  deals  with  the  in- 
heritance of  the  weight  of  beans,  but  as  an  account  of 
these  experiments  would  involve  us  in  the  consideration  of 
a  large  amount  of  detail  we  may  take  a  simple  imaginary 
case  to  illustrate  the  nature  of  the  conclusions  at  which  he 
arrived.  If  we  weigh  a  number  of  seeds  collected  from  a 
patch  of  plants  such  as  Johannsen's  beans  we  should  find 
that  they  varied  considerably  in  size.  The  majority 
would  probably  not  diverge  very  greatly  from  the  general 
average,  and  as  we  approached  the  high  or  low  extreme 
we  should  find  a  constantly  decreasing  number  of  in- 
dividuals with  these  weights.  Let  us  suppose  that  the 
weight  of  our  seed  varied  between  4  and  20  grains,  that 
the  greatest  number  of  seeds  were  of  the  mean  weight,  viz. 
12  grains,  and  that  as  we  passed  to  either  extreme  at  4 
and  20  the  number  became  regularly  less.  The  weight 
relation  of  such  a  collection  of  seeds  can  be  expressed  by 
the  accompanying  curve  (Fig.  30) .  Now  if  we  select  for 


XIV 


ECONOMICAL 


161 


sowing  only  that  seed  which  weighs  over  1 2  grains,  we  shall 
find  that  in  the  next  generation  the  average  weight  of  the 
seed  is  raised  and  the  curve  becomes  somewhat  shifted  to 
the  right  as  in  the  dotted  line  of  Fig.  30.  By  continually 
selecting  we  can  shift  our  curve  a  little  more  to  the  right, 
i.e.  we  can  increase  the  average  weight  of  the  seeds  until 
at  last  we  come  to  a  limit  beyond  which  further  selection 
has  no  effect.  This  phenomenon  has  been  long  known, 
and  it  was  customary  to  regard  these  variations  as  of  a 
continuous  nature,  i.e.  as  all  chance  fluctuations  in  a 
homogeneous  mass,  and  the  effect  of  selection  was  sup- 


4        6       8       10      12      14     16      18      20 
Weight  of  individual  seeds 

FIG.  31. 
Curves  to  illustrate  the  conception  of  pure  lines  in  a  population. 

posed  to  afford  evidence  that  small  continuous  variations 
could  be  irlcreased  by  this  process.  But  Johannsen's 
results  point  to  another  interpretation.  Instead  of  our 
material  being  homogeneous  it  is  probably  a  mixture  of 
several  strains  each  with  its  own  average  weight  about 


162  MENDELISM  CHAP. 

which  the  varying  conditions  of  the  environment  cause  it 
to  fluctuate.  Each  of  these  strains  is  termed  a  pure  line. 
If  we  imagine  that  there  are  three  such  pure  lines  in  our 
imaginary  case,  with  average  weights  10,  12,  14  grains 
respectively,  and  if  the  range  of  fluctuation  of  each  of 
these  pure  lines  is  1 2  grains,  then  our  curve  must  be  repre- 
sented as  made  up  of  the  three  components 

A  fluctuating  between  4  and  16  with  a  mean  of  10 
B          „  ,,6     „     18     „  „  12 

»  »  8       ,,       20        ,,  ,,  14 

as  is  shown  in  Fig.  31.  A  seed  that  weighs  12  grains  may 
belong  to  any  of  these  three  strains.  It  may  be  an  aver- 
age seed  of  B,  or  a  rather  large  seed  of  A,  or  a  rather  small 
seed  of  C.  If  it  belongs  to  B  its  offspring  will  average  12 
grains,  if  to  A  they  will  average  10  grains,  and  if  to  C 
they  will  average  14  grains.  Seeds  of  similar  weight 
may  give  a  different  result  because  they  happen  to  be 
fluctuations  of  different  pure  lines.  But  within  the  pure 
line  any  seed,  large  or  small,  produces  the  average  result 
for  that  line.  Thus  a  seed  of  line  C  which' weighs  20 
grains  will  give  practically  the  same  result  as  one  that 
weighs  10  grains. 

On  this  view  we  can  understand  why  selection  of  the 
largest  seed  raises  the  average  weight  in  the  next  genera- 
tion. We  are  picking  out  more  of  C  and  less  of  A  and  B, 
and  as  this  process  is  repeated  the  proportion  of  C  gradu- 
ally increases  and  we  get  the  appearance  of  selection  act- 


xiv  ECONOMICAL  163 

ing  on  a  continuously  varying  homogeneous  material  and 
producing  a  permanent  effect.  This  is  because  the  in- 
terval between  the  average  weight  of  the  different  pure 
lines  is  small  compared  with  the  environmental  fluctua- 
tions. None  the  less  it  is  there,  and  the  secret  of  separat- 
ing and  fixing  any  of  these  pure  lines  is  again  to  breed  from 
the  individual  separately.  As  soon  as  the  pure  line  is 
separated  further  selection  becomes  superfluous. 

Since  the  publication  of  Darwin's  famous  work  upon 
the  effects  of  cross  and  self  fertilisation,  it  has  been 
generally  accepted  that  the  effect  of  a  cross  is  commonly, 
though  not  always,  to  introduce  fresh  vigour  into  the 
offspring,  though  why  this  should  be  so  we  are  quite  at  a 
loss  to  explain.  Continued  close  inbreeding,  on  the  con- 
trary, eventually  leads  to  deterioration,  though,  as  in 
many  self -fertilised  plants,  a  considerable  number  of  gen- 
erations may  elapse  before  it  shows  itself  in  any  marked 
degree.  The  fine  quality  of  many  of  the  seedsman's 
choice  varieties  of  vegetables  probably  depends  upon  the 
fact  that  they  had  resulted  from  a>  cross  but  a  few 
generations  back,  and  it  is  possible  that  they  often 
oust  the  older  kinds  not  because  they  started  as  some- 
thing intrinsically  better,  but  because  the  latter  had 
gradually  deteriorated  through  continuous  self-fertilisa- 
tion. Most  breeders  are  fully  alive  to  the  beneficial  re- 
sults of  a  cross  so  far  as  vigour  is  concerned,  but  they 
often  hesitate  to  embark  upon  it  owing  to  what  was  held 


1 64  MENDELISM  CHAP. 

to  be  the  inevitably  lengthy  and  laborious  business  of  re- 
covering the  original  variety  and  refixing  it,  even  if  in  the 
process  it  was  not  altogether  lost.  That  danger  Mendel- 
ism  has  removed,  and  we  now  know  that  by  working  on 
these  lines  it  is  possible  in  three  or  four  generations  to 
recover  the  original  variety  in  a  fixed  state  with  all  the 
superadded  vigour  that  follows  from  a  cross. 

Nor  is  the  problem  one  that  concerns  self -fertilised 
plants  only.  Plants  that  are  reproduced  asexually  often 
appear  to  deteriorate  after  a  few  generations  unless  a  sex- 
ual generation  is  introduced.  New  varieties  of  potato, 
for  example,  are  frequently  put  upon  the  market,  and 
their  excellent  qualities  give  them  a  considerable  vogue. 
Much  is  expected  of  them,  but  time  after  time  they  de- 
teriorate in  a  disappointing  way  and  are  lost  to  sight.  It 
is  not  improbable  that  we  are  here  concerned  with  a  case 
in  which  the  plants  lose  their  vigour  after  a  few  asexual 
generations  of  reproduction  from  tubers,  and  can  only 
recover  it  with  the  stimulus  that  results  from  the  inter- 
polation of  a  sexual  generation.  Unfortunately  this 
generally  means  that  the  variety  is  lost,  for  owing  to  the 
haphazard  way  in  which  new  kinds  of  potatoes  are  repro- 
duced it  is  probable  that  most  cultivated  varieties  are 
complex  heterozygotes.  Were  the  potato  plant  subjected 
to  careful  analysis  and  the  various  factors  determined 
upon  which  its  variations  depend,  we  should  be  in  a  posi- 
tion to  remake  continually  any  good  potato  without 


xiv  ECONOMICAL  165 

running  the  risk  of  losing  it  altogether,  as  is  now  so  often 
the  case. 

The  application  of  Mendelian  principles  is  likely  to 
prove  of  more  immediate  service  for  plants  than  animals, 
for  owing  to  the  large  numbers  which  can  be  rapidly 
raised  from  a  single  individual  and  the  prevalence  of  self- 
fertilisation,  the  process  of  analysis  is  greatly  simplified. 
Even  apart  from  the  circumstance  that  the  two  sexes 
may  sometimes  differ  in  their  powers  of  transmission,  the 
mere  fact  of  their  separation  renders  the  analysis  of  their 
properties  more  difficult.  And  as  the  constitution  of  the 
individual  is  determined  by  the  nature  and  quality  of  its 
offspring,  it  is  not  easy  to  obtain  this  knowledge  where 
the  offspring,  as  in  most  animals,  are  relatively  few. 
Still,  as  has  been  abundantly  shown,  the  same  principles 
hold  good  here  also,  and  there  is  no  reason  why  the  pro- 
cess of  analysis,  though  more  troublesome,  should  not  be 
effectively  carried  out.  At  the  same  time,  it  affords  the 
breeder  a  rational  basis  for  some  familiar  but  puzzling 
phenomena.  The  fact,  for  instance,  that  certain  characters 
often  "  skip  a  generation  "  is  simply  the  effect  of  dominance 
in  F!  and  the  reappearance  of  the  recessive  character  in 
the  following  generation.  " Reversion"  and  " atavism," 
again,  are  phenomena  which  are  no  longer  mysterious, 
but  can  be  simply  expressed  in  Mendelian  terms  as  we 
have  already  suggested  in  Chap.  VI.  The  occasional 
appearance  of  a  sport  in  a  supposedly  pure  strain  is 


i66  MENDELISM  CHAP. 

often  due  to  the  reappearance  of  a  recessive  character. 
Thus  even  in  the  most  highly  pedigreed  strains  of  polled 
cattle  such  as  the  Aberdeen  Angus,  occasional  individuals 
with  horns  appear.  The  polled  character  is  dominant  to 
the  horned,  and  the  occasional  reappearance  of  the  horned 
animal  is  due  to  the  fact  that  some  of  the  polled  herd  are 
heterozygous  in  this  character.  When  two  such  indi- 
viduals are  mated,  the  chances  are  i  in  4  that  the  offspring 
will  be  horned.  Though  the  heterozygous  individuals  may 
be  indistinguishable  in  appearance  from  the  pure  domi- 
nant, they  can  be  readily  separated  by  the  breeding  test. 
For  when  crossed  by  the  recessive,  in  this  case  horned  ani- 
mals, the  pure  dominant  gives  only  polled  beasts,  while 
the  heterozygous  individual  gives  equal  numbers  of 
polled  and  horned  ones.  In  this  particular  instance  it 
would  probably  be  impracticable  to  test  all  the  cows  by 
crossing  with  a  horned  bull.  For  in  each  case  it  would  be 
necessary  to  have  several  polled  calves  from  each  before 
they  could  with  reasonable  certainty  be  regarded  as  pure 
dominants.  But  to  ensure  that  no  horned  calves  should 
come,  it  is  enough  to  use  a  bull  which  is  pure  for  that 
character.  This  can  easily  be  tested  by  crossing  him 
with  a  dozen  or  so  horned  cows.  If  he  gets  no  horned 
calves  out  of  these  he  may  be  regarded  as  a  pure  dominant 
and  thenceforward  put  to  his  own  cows,  whether  horned 
or  polled,  with  the  certainty  that  all  his  calves  will  be 
polled. 


xiv  ECONOMICAL  167 

Or,  again,  suppose  that  a  breeder  has  a  chestnut  mare 
and  wishes  to  make  certain  of  a  bay  foal  from  her.  We 
know  that  bay  is  dominant  to  chestnut,  and  that  if  a 
homozygous  bay  stallion  is  used  a  bay  foal  must  result. 
In  his  choice  of  a  sire,  therefore,  the  breeder  must  be 
guided  by  the  previous  record  of  the  animal,  and  select 
one  that  has  never  given  anything  but  bays  when  put  to 
either  bay  or  chestnut  mares.  In  this  way  he  will  assure 
himself  of  a  bay  foal  from  his  chestnut  mare,  whereas  if 
the  record  of  the  sire  shows  that  he  has  given  chestnuts 
he  will  be  heterozygous,  and  the  chances  of  his  getting  a 
bay  or  a  chestnut  out  of  a  chestnut  mare  are  equal. 

It  is  not  impossible  that  the  breeder  may  be  unwilling 
to  test  his  animals  by  crossing  them  with  a  different  breed 
through  fear  that  their  purity  may  be  thereby  impaired, 
and  that  the  influence  of  the  previous  cross  may  show 
itself  in  succeeding  generations.  He  might  hesitate,  for 
instance,  to  test  his  polled  cows  by  crossing  them  with  a 
horned  bull  for  fear  of  getting  horned  calves  when  the 
cows  were  afterwards  put  to  a  polled  bull  of  their  own 
breed.  The  belief  in  the  power  of  a  sire  to  influence  sub- 
sequent generations,  or  telegony  as  it  is  sometimes  called, 
is  not  uncommon  even  to-day.  Nevertheless,  carefully 
conducted  experiments  by  more  than  one  competent 
observer  have  failed  to  elicit  a  single  shred  of  unequiv- 
ocal evidence  in  favour  of  the  view.  Until  we  have 
evidence  based  upon  experiments  which  are  capable  of 


i68  MENDELISM  CHAP. 

repetition,  we  may  safely  ignore  telegony  as  a  factor  in 
heredity. 

Heterozygous  forms  play  a  greater  part  in  the  breeding 
of  animals  than  of  plants,  for  many  of  the  qualities  sought 
after  by  the  breeder  are  of  this  nature.  Such  is  the  blue 
of  the  Andalusian  fowl,  and,  according  to  Professor  Wil- 
son, the  roan  of  the  Shorthorn  is  similar,  being  the  hetero- 
zygous form  produced  by  mating  red  with  white.  The 
characters  of  certain  breeds  of  canaries  and  pigeons  again 
appear  to  depend  upon  their  heterozygous  nature.  Such 
forms  cannot,  of  course,  ever  be  bred  true,  and  where  sev- 
eral factors  are  concerned  they  may  when  bred  together 
produce  but  a  small  proportion  of  offspring  like  them- 
selves. As  soon,  however,  as  their  constitution  has  been 
analysed  and  expressed  in  terms  of  Mendelian  factors, 
pure  strains  can  be  built  up  which  when  crossed  will  give 
nothing  but  offspring  of  the  desired  heterozygous  form. 

The  points  with  which  the  breeder  is  concerned  are 
often  fine  ones,  not  very  evident  except  to  the,  practised 
eye.  Between  an  ordinary  Dutch  rabbit  and  a  winner, 
or  between  the  comb  of  a  Hamburgh  that  is  fit  to  show 
and  one  that  is  not,  the  differences  are  not  very  apparent 
to  the  uninitiated.  Whether  Mendelism  will  assist  the 
breeder  in  the  production  of  these  finer  points  is  at  present 
doubtful.  It  may  be  that  these  small  differences  are 
heritable,  such  as  those  that  form  the  basis  of  Johann- 
sen's  pure  lines.  In  this  case  the  breeder's  outlook  is 


xiv  ECONOMICAL  169 

hopeful.  But  it  may  be  that  the  variations  which  he 
seeks  to  perpetuate  are  of  the  nature  of  fluctuations,  de- 
pendent upon  the  earlier  life  conditions  of  the  individual, 
and  not  upon  the  constitution  of  the  gametes  by  which 
it  was  formed.  If  such  is  the  case,  he  will  get  no  help 
from  the  science  of  heredity,  for  we  know  of  no  evidence 
which  might  lead  us  to  suppose  that  variations  of  this 
sort  can  ever  become  fixed  and  heritable. 


CHAPTER  XV 

MAN 

THOUGH  the  interest  attaching  to  heredity  in  man  is 
more  widespread  than  in  other  animals,  it  is  far  more 
difficult  to  obtain  evidence  that  is  both  complete  and  ac- 
curate. The  species  is  one  in  which  the  differentiating 
characters  separating  individual  from  individual  are  very 
numerous,  while  the  number  of  the  offspring  is  compara- 
tively few,  and  the  generations  are  far  between.  For 
these  reasons,  even  if  it  were  possible,  direct  experimen- 
tal work  with  man  would  be  likely  to  prove  both  tedious 
and  expensive.  There  is,  however,  another  method  be- 
sides the  direct  one  from  which  something  can  be  learned. 
This  consists  in  collecting  all  the  evidence  possible,  ar- 
ranging it  in  the  form  of  pedigrees,  and  comparing  it  with 
standard  cases  already  worked  out  in  animals  and  plants. 
In  this  way  it  has  been  possible  to  demonstrate  in  man 
the  existence  of  several  characters  showing  simple  Men- 
delian  inheritance.  As  few  besides  medical  men  have 
hitherto  been  concerned  practically  with  heredity,  such 
records  as  exist  are,  for  the  most  part,  records  of  deform- 
ity or  of  disease.  So  it  happens  that  most  of  the  pedi- 

170 


xv  MAN  171 

grees  at  present  available  deal  with  characters  which  are 
usually  classed  as  abnormal.  In  some  of  these  the  in- 
heritance is  clearly  Mendelian.  One  of  the  cases  which 


FIG.  32. 

Normal  and  brachydactylous  hands  placed  together  for  comparison. 
(From  Drink  water.) 

has  been  most  fully  worked  out  is  that  of  a  deformity 
known  as  brachydactyly.     In  brachydactylous  people  the 


172  MENDELISM  CHAP. 

whole  of  the  body  is  much  stunted,  and  the  fingers  and 
toes  appear  to  have  two  joints  only  instead  of  three  (cf. 
Figs.  32  and  33).  The  inheritance  of  this  peculiarity  has 
been  carefully  investigated  by  Dr.  Drinkwater,  who  col- 
lected all  the  data  he  was  able  to  find  among  the  members 
of  a  large  family  in  which  it  occurred.  The  result  is  the 


FIG.  33. 

Radiograph  of  a  brachydactylous  hand.  ->. 

pedigree  shown  on  p.  173.  It  is  assumed  that  all  who 
are  recorded  as  having  offspring  were  married  to  normals. 
Examination  of  the  pedigree  brings  out  the  facts  (i)  that 
all  affected  individuals  have  an  affected  parent ;  (2)  that 
none  of  the  unaffected  individuals,  though  sprung  from 
the  affected,  ever  have  descendants  who  are  affected,  and 
(3)  that  in  families  where  both  affected  and  unaffected 


XV 


MAN 


173 


occur,  the  numbers  of 
the  two  classes  are,  on 
the  average,  equal. 
(The  sum  of  such 
families  in  the  com- 
plete pedigree  is 
thirty-nine  affected 
and  thirty-six  nor- 
mals.) It  is  obvious 
that  these  are  the 
conditions  which  are 
fulfilled  in  a  simple 
Mendelian  case,  and 
there  is  nothing  in  this 
pedigree  to  contradict 
the  assertion  that 
brachydactyly,  what- 
ever it  may  be  due  to, 
behaves  as  a  simple 
dominant  to  the  nor- 
mal form,  i.e.  that  it  ^ 

depends  upon  a  factor 
which  the  normal  does 
not  contain.  The  re- 
cessive normals  can- 
not transmit  the 
affected  condition 
whatever  their  an- 


174  MENDELISM  CHAP. 

cestry.  Once  free  they  are  always  free,  and  can  marry 
other  normals  with  full  confidence  that  none  of  their 
children  will  show  the  deformity. 

The  evidence  available  from  pedigrees  has  revealed 
the  simplest  form  of  Mendelian  inheritance  in  several 
human  defects  and  diseases,  among  which  may  be  men- 
tioned presenile  cataract  of  the  eyes,  an  abnormal  form 
of  skin  thickening  in  the  palms  of  the  hands  and  soles 
of  the  feet,  known  as  tylosis,  and  epidermolysis  bullosa, 
a  disease  in  which  the  skin  rises  up  into  numerous  burst- 
ing blisters. 

Among  the  most  interesting  of  all  human  pedigrees  is, 
one  recently  built  up  by  Mr.  Nettleship  from  the  records 
of  a  night-blind  family  living  near  Monpelier  in  the  south 
of  France.  In  night-blind  people  the  retina  is  insensitive 
to  light  which  falls  below  a  certain  intensity,  and  such 
people  are  consequently  blind  in  failing  daylight  or  in 
moonlight.  As  the  Monpelier  case  had  excited  interest 
for  some  time,  the  records  are  unusually  complete.  They 
commence  with  a  certain  Jean  Nougaret,  who  was  born  in 
1637,  and  suffered  from  night-blindness,  and  they  end  for 
the  present  with  children  who  are  to-day  but  a  few  years 
of  age.  Particulars  are  known  of  over  2000  of  the  de- 
scendants of  Jean  Nougaret.  Through  ten  generations 
and  nearly  three  centuries  the  affection  has  behaved  as 
a  Mendelian  dominant,  and  there  is  no  sign  that  long- 
continued  marriage  with  folk  of  normal  vision  has  pro- 
duced any  amelioration  of  the  night-blind  state. 


xv  MAN  175 

Besides  cases  such  as  these  where  a  simple  form  of 
Mendelian  inheritance  is  obviously  indicated,  there  are 
others  which  are  more  difficult  to  read.  Of  some  it  may 
be  said  that  on  the  whole  the  peculiarity  behaves  as 
though  it  were  an  ordinary  dominant ;  but  that  exceptions 
occur  in  which  affected  children  are  born  to  unaffected 
parents.  It  is  not  impossible  that  the  condition  may, 
like  colour  in  the  sweet  pea,  depend  upon  the  presence  or 


rf  i  ^9  9     9       9          9  -rf   *    9    9      9     ^ 

1    i )    i     ill  r~i   i 

Hi 


FIG.  35- 

Pedigree  of    a  haemophilic  family.    Affected  (all  males)  represented  by  black,  and 
normals  of  both  sexes  by  light  circles.     (From  Stahel.) 

absence  of  more  than  one  factor.  In  none  of  these  cases, 
however,  are  the  data  sufficient  for  determining  with  cer- 
tainty whether  this  is  so  or  not. 

A  group  of  cases  of  exceptional  interest  is  that  in  which 
the  incidence  of  disease  is  largely,  if  not  absolutely,  re- 
stricted to  one  sex,  and  so  far  as  is  hitherto  known  the 
burden  is  invariably  borne  by  the  male.  In  the  inheri- 
tance of  colour-blindness  (p.  117)  we  have  already  dis- 
cussed an  instance  in  which  the  defect  is  rare,  though  not 


176  MENDELISM  CHAP. 

unknown,  in  the  female.  Sex-limited  inheritance  of  a 
similar  nature  is  known  for  one  or  two  ocular  defects,  and 
for  several  diseases  of  the  nervous  system.  In  the  pe- 
culiarly male  disease  known  as  haemophilia  the  blood  re- 
fuses to  clot  when  shed,  and  there  is  nothing  to  prevent 
great  loss  from  even  a  superficial  scratch.  In  its  general 
trend  the  inheritance  of  haemophilia  is  not  unlike  that  of 
horns  among  sheep,  and  it  is  possible  that  we  are  here 
again  dealing  with  a  character  which  is  dominant  in  one 
sex  and  recessive  in  the  other.  But  the  evidence  so  far 
collected  points  to  a  difference  somewhere,  for  in  haemo- 
philic  families  the  affected  males,  instead  of  being  equal 
in  number  to  the  unaffected,  show  a  considerable  prepon- 
derance. The  unfortunate  nature  of  the  defect,  however, 
forces  us  to  rely  for  our  interpretation  almost  entirely 
upon  the  families  produced  by  the  unaffected  females 
who  can  transmit  it.  Our  knowledge  of  the  offspring  of 
"  bleeding  "males  is  as  yet  far  too  scanty,  and  until  it  is 
improved,  or  until  we  can  find  some  parallel  case  in  ani- 
mals or  plants,  the  precise  scheme  of  inheritance  for 
haemophilia  must  remain  undecided. 

Though  by  far  the  greater  part  of  the  human  evidence 
relates  to  abnormal  or  diseased  conditions,  a  start  has 
been  made  in  obtaining  pedigrees  of  normal  characters. 
From  the  ease  with  which  it  can  be  observed,  it  was 
natural  that  eye-colour  should  be  early  selected  as  a 
subject  of  investigation,  and  the  work  of  Hurst  and  others 


xv  MAN  177 

has  clearly  demonstrated  the  existence  of  one  Mendelian 
factor  in  operation  here.  Eyes  are  of  many  colours,  and 
the  colour  depends  upon  the  pigment  in  the  iris.  Some 
eyes  have  pigment  on  both  sides  of  the  iris  —  on  the  side 
that  faces  the  retina  as  well  as  on  the  side  that  looks  out 
upon  the  world.  Other  eyes  have  pigment  on  the  retinal 
side  only.  To  this  class  belong  the  blues  and  clear  greys ; 
while  the  eyes  with  pigment  in  front  of  the  iris  also  are 
brown,  hazel,  or  green  in  various  shades  according  to  the 
amount  of  pigment  present.  In  albino  animals  the  pig- 
ment is  entirely  absent,  and  as  the  little  blood-vessels  are 
not  obscured  the  iris  takes  on  its  characteristic  pinkish-  ^ 
red  appearance.  The  condition  in  which  pigment  is  , 
present  in  front  of  the  iris  is  dominant  to  that  in  which  .^A 
it  is  absent.  Greens,  browns,  or  hazels  mated  together 
may,  if  heterozygous,  give  the  recessive  blue,  but  no  in- 
dividuals of  the  brown  class  are  to  be  looked  for  among 
the  offspring  of  blues  mated  together.  The  blues,  how- 
ever, may  carry  factors  which  are  capable  of  modifying  the 
brown.  Just  as  the  pale  pink- tinged  sweet  pea  (PI.  IV.,  9) 
when  mated  with  a  suitable  white  gives  only  deep  purples, 
so  an  eye  with  very  little  brown  pigment  mated  with  cer- 
tain blues  produces  progeny  of  a  deep  brown,  far  darker 
than  either  parent.  The  blue  may  carry  a  factor  which 
brings  about  intensification  of  the  brown  pigment. 
There  are  doubtless  other  factors  which  modify  the  brown 
when  present,  but  we  do  not  yet  know  enough  of  the  in- 


178  MENDELISM  CHAP. 

heritance  of  the  various  shades  to  justify  any  statement 
other  than  that  the  heredity  of  the  pigment  in  front  of 
the  iris  behaves  as  though  it  were  due  to  a  Mendelian 
factor. 

Even  this  fact  is  of  considerable  importance,  for  it  at 
once  suggests  that  the  present  systems  of  classification  of 
eye-colours,  to  which  some  anthropologists  attach  con- 
siderable weight,  are  founded  on  a  purely  empirical  and 
unsatisfactory  basis.  Intensity  of  colour  is  the  criterion 
at  present  in  vogue,  and  it  is  customary  to  arrange  the 
eye-colours  in  a  scale  of  increasing  depth  of  shade,  start- 
ing with  pale  greys  and  ending  with  the  deepest  browns. 
On  this  system  the  lighter  greens  are  placed  among  the 
blues.  But  we  now  know  that  blues  may  differ  from  the 
deep  browns  in  the  absence  of  only  a  single  factor,  while, 
on  the  other  hand,  the  difference  between  a  blue  and  a 
green  may  be  a  difference  dependent  upon  more  than  one 
factor.  To  what  extent  eye-colour  may  be  valuable  as  a 
criterion  of  race  it  is  at  present  impossible  to  say,  but  if  it 
is  ever  to  become  so,  it  will  only  be  after  a  searching  Men- 
delian analysis  has  disclosed  the  factors  upon  which  the 
numerous  varieties  depend. 

A  discussion  of  eye-colour  suggests  reflections  of  another 
kind.  It  is  difficult  to  believe  that  the  markedly  different 
states  of  pigmentation  which  occur  in  the  same  species 
are  not  associated  with  deep-seated  chemical  differences 
influencing  the  character  and  bent  of  the  individual. 


xv  MAN  179 

May  not  these  differences  in  pigmentation  be  coupled 
with  and  so  become  in  some  measure  a  guide  to  mental 
and  temperamental  characteristics?  In  the  National 
Portrait  Gallery  in  London  the  pictures  of  celebrated  men 
and  women  are  largely  grouped  according  to  the  vocations 
in  which  they  have  succeeded.  The  observant  will 
probably  have  noticed  that  there  is  a  tendency  for  a  given 
type  of  eye-colour  to  predominate  in  some  of  the  larger 
groups.  It  is  rare  to  find  anything  but  a  blue  among  the 
soldiers  and  sailors,  while  among  the  actors,  preachers, 
and  orators  the  dark  eye  is  predominant,  although  for  the 
population  as  a  whole  it  is  far  scarcer  than  the  light.  The 
facts  are  suggestive,  and  it  is  not  impossible  that  future 
research  may  reveal  an  intimate  connection  between  pecu- 
liarities of  pigmentation  and  peculiarities  of  mind. 

The  inheritance  of  mental  characters  is  often  elusive, 
for  it  is  frequently  difficult  to  appraise  the  effects  of  early 
environment  in  determining  a  man's  bent.  That  ability 
can  be  transmitted  there  is  no  doubt,  for  this  is  borne  out 
by  general  experience,  as  well  as  by  the  numerous  cases  of 
able  families  brought  together  by  Galton  and  others. 
But  when  we  come  to  inquire  more  precisely  what  it  is 
that  is  transmitted  we  are  baffled.  A  distinguished  son 
follows  in  the  footsteps  of  a  distinguished  father.  Is  this 
due  to  the  inheritance  of  a  particular  mental  aptitude,  or 
is  it  an  instance  of  general  mental  ability  displayed  in  a 
field  rendered  attractive  by  early  association  ?  We  have 


i8o  MENDELISM  CHAP. 

at  present  very  little  definite  evidence  for  supposing  that 
what  appear  to  be  special  forms  of  ability  may  be  due  to 
specific  factors.  Hurst,  indeed,  has  brought  forward 
some  facts  which  suggest  that  musical  sense  sometimes  be- 
haves as  a  recessive  character,  and  it  is  likely  that  the 
study  of  some  clean-cut  faculty  such  as  the  mathematical 
one  would  yield  interesting  results. 

The  analysis  of  mental  characters  will  no  doubt  be 
very  difficult,  and  possibly  the  best  line  of  attack  is  to 
search  for  cases  where  they  are  associated  with  some 
physical  feature  such  as  pigmentation.  If  an  association 
of  this  kind  be  found,  and  the  pigmentation  factors  be 
determined,  it  is  evident  that  we  should  thereby  obtain 
an  insight  into  the  nature  of  the  units  upon  which  mental 
conditions  depend.  Nor  must  it  be  forgotten  that  men- 
tal qualities,  such  as  quickness,  generosity,  instability, 
etc.,  —  qualities  which  we  are  accustomed  to  regard  as 
convenient  units  in  classifying  the  different  minds  with 
which  we  are  daily  brought  into  contact,  —  are  not 
necessarily  qualities  that  correspond  to  heritable  units. 
Effective  mental  ability  is  largely  a  matter  of  tempera- 
ment, and  this  in  turn  is  quite  possibly  dependent  upon 
the  various  secretions  produced  by  the  different  tissues 
of  the  body.  Similar  nervous  systems  associated  with 
different  livers  might  conceivably  result  in  individuals 
upon  whose  mental  ability  the  world  would  pass  a  very 
different  judgment.  Indeed,  it  is  not  at  all  impossible 


xv  MAN  181 

that  a  particular  form  of  mental  ability  may  depend  for 
its  manifestation,  not  so  much  upon  an  essential  difference 
in  the  structure  of  the  nervous  system,  as  upon  the  pro- 
duction by  another  tissue  of  some  specific  poison  which 
causes  the  nervous  system  to  react  in  a  definite  way. 
We  have  mentioned  these  possibilities  merely  to  indicate 
how  complex  the  problem  may  turn  out  to  be.  Though 
there  is  no  doubt  that  mental  ability  is  inherited,  what  it 
is  that  is  transmitted,  whether  factors  involving  the 
quality  and  structure  of  the  nervous  system  itself,  or 
factors  involving  the  production  of  specific  poisons  by 
other  tissues,  or  both  together,  is  at  present  uncertain. 
Little  as  is  known  to-day  of  heredity  in  man,  that  little 
is  of  extraordinary  significance.  The  qualities  of  men 
and  women,  physical  and  mental,  depend  primarily  upon 
the  inherent  properties  of  the  gametes  which  went  to 
their  making.  Within  limits  these  qualities  are  elastic, 
and  can  be  modified  to  a  greater  or  lesser  extent  by  in- 
fluences brought  to  bear  upon  the  growing  zygote,  pro- 
vided always  that  the  necessary  basis  is  present  upon 
which  these  influences  can  work.  If  the  mathematical 
faculty  has  been  carried  in  by  the  gamete,  the  education 
of  the  zygote  will  enable  him  to  make  the  most  of  it. 
But  if  the  basis  is  not  there,  no  amount  of  education  can 
transform  that  zygote  into  a  mathematician.  This  is  a 
matter  of  common  experience.  Neither  is  there  any 
reason  for  supposing  that  the  superior  education  of  a 


182  MENDELISM  CHAP. 

mathematical  zygote  will  thereby  increase  the  mathe- 
matical propensities  of  the  gametes  which  live  within 
him.  For  the  gamete  recks  little  of  quaternions.  It  is 
true  that  there  is  progress  of  a  kind  in  the  world,  and 
that  this  progress  is  largely  due  to  improvements  in 
education  and  hygiene.  The  people  of  to-day  are  better 
fitted  to  cope  with  their  material  surroundings  than  were 
the  people  of  even  a  few  thousand  years  ago.  And  as 
time  goes  on  they  are  able  more  and  more  to  control  the 
workings  of  the  world  around  them.  But  there  is  no  rea- 
son for  supposing  that  this  is  because  the  effects  of  educa- 
tion are  inherited.  Man  stores  knowledge  as  a  bee  stores 
honey  or  a  squirrel  stores  nuts.  With  man,  however,  the 
hoard  is  of  a  more  lasting  nature.  Each  generation  in 
using  it  sifts,  adds,  and  rejects,  and  passes  it  on  to  the 
next  a  little  better  and  a  little  fuller.  When  we  speak  of 
progress  we  generally  mean  that  the  hoard  has  been  im- 
proved, and  is  of  more  service  to  man  in  his  attempts  to 
control  his  surroundings.  Sometimes  this  hoarded  know- 
ledge is  spoken  of  as  the  inheritance  which  a  generation 
receives  from  those  who  have  gone  before.  This  is  mis- 
leading. The  handing  on  of  such  knowledge  has  nothing 
more  to  do  with  heredity  in  the  biological  sense  than  has 
the  handing  on  from  parent  to  offspring  of  a  picture,  or 
a  title,  or  a  pair  of  boots.  All  these  things  are  but  the 
transfer  from  zygote  to  zygote  of  something  extrinsic  to 
the  species.  Heredity,  on  the  other  hand,  deals  with  the 


xv  MAN  183 

transmission  of  something  intrinsic  from  gamete  to  zygote 
and  from  zygote  to  gamete.  It  is  the  participation  of  the 
gamete  in  the  process  that  is  our  criterion  of  what  is  and 
what  is  not  heredity. 

Better  hygiene  and  better  education,  then,  are  good 
for  the  zygote,  because  they  help  him  to  make  the  fullest 
use  of  his  inherent  qualities.  But  the  qualities  them- 
selves remain  unchanged  in  so  far  as  the  gamete  is  con- 
cerned, since  the  gamete  pays  no  heed  to  the  intellectual 
development  of  the  zygote  in  whom  he  happens  to  dwell. 
Nevertheless,  upon  the  gamete  depend  those  inherent 
faculties  which  enable  the  zygote  to  profit  by  his  oppor- 
tunities, and,  unless  the  zygote  has  received  them  from 
the  gamete,  the  advantages  of  education  are  of  little 
worth.  If  we  are  bent  upon  producing  a  permanent  bet- 
terment that  shall  be  independent  of  external  circum- 
stances, if  we  wish  the  national  stock-to  become  inherently 
more  vigorous  in  mind  and  body,  more  free  from  con- 
genital physical  defect  and  feeble  mentality,  better  able 
to  assimilate  and  act  upon  the  stores  of  knowledge  which 
have  been  accumulated  through  the  centuries,  then  it  is 
the  gamete  that  we  must  consult.  The  saving  grace  is 
with  the  gamete,  and  with  the  gamete  alone. 

People  generally  look  upon  the  human  species  as  having 
two  kinds  of  individuals,  males  and  females,  and  it  is  for 
them  that  the  sociologists  and  legislators  frame  their 
schemes.  This,  however,  is  but  an  imperfect  view  to 


i84  MENDELISM  CHAP. 

take  of  ourselves.  In  reality  we  are  of  four  kinds,  male 
zygotes  and  female  zygotes,  large  gametes  and  small 
gametes,  and  heredity  is  the  link  that  binds  us  together. 
If  our  lives  were  like  those  of  the  starfish  or  the  sea-urchin, 
we  should  probably  have  realised  this  sooner.  For  the 
gametes  of  these  animals  live  freely,  and  contract  their 
marriages  in  the  waters  of  the  sea.  With  us  it  is  different, 
because  half  of  us  must  live  within  the  other  half  or 
perish.  Parasites  upon  the  rest,  levying  a  daily  toll 
of  nutriment  upon  their  hosts,  they  are  yet  in  some  meas- 
ure the  arbiters  of  the  destiny  of  those  within  whom  they 
dwell.  At  the  moment  of  union  of  two  gametes  is  de- 
cided the  character  of  another  zygote,  as  well  as  the 
nature  of  the  population  of  gametes  which  must  make  its 
home  within  him.  The  union  once  affected  the  inevitable 
sequence  takes  its  course,  and  whether  it  be  good,  or 
whether  it  be  evil,  we,  the  zygotes,  have  no  longer  power 
to  alter  it.  We  are  in  the  hands  of  the  gamete ;  yet  not 
entirely.  For  though  we  cannot  influence  their  behaviour 
we  can  nevertheless  control  their  unions  if  we  choose  to  do 
so.  By  regulating  their  marriages,  by  encouraging  the 
desirable  to  come  together,  and  by  keeping  the  undesir- 
able apart  we  could  go  far  towards  ridding  the  world  of  the 
squalor  and  the  misery  that  come  through  disease  and 
weakness  and  vice.  But  before  we  can  be  prepared  to  act, 
except,  perhaps,  in  the  simples  cases,  we  must  learn  far 
more  about  them.  At  present  we  are  woefully  ignorant 


xv  MAN  185 

of  much,  though  we  do  know  that  full  knowledge  is  largely 
a  matter  of  time  and  means.  One  day  we  shall  have  it, 
and  the  day  may  be  nearer  than  most  suspect.  Whether 
we  make  use  of  it  will  depend  in  great  measure  upon 
whether  we  are  prepared  to  recognise  facts,  and  to  modify 
or  even  destroy  some  of  the  conventions  which  we  have 
become  accustomed  to  regard  as  the  foundations  of  our 
social  life.  Whatever  be  the  outcome,  there  can  be  little 
doubt  that  the  future  of  our  civilisation,  perhaps  even  the 
possibility  of  a  future  at  all,  is  wrapped  up  with  the 
recognition  we  accord  to  those  who  live  unseen  and  in- 
articulate within  us  —  the  fateful  race  of  gametes  so 
irrevocably  bound  to  us  by  that  closest  of  all  ties,  heredity. 


APPENDIX 

As  some  readers  may  possibly  care  to  repeat  Mendel's 
experiments  for  themselves,  a  few  words  on  the  methods 
used  in  crossing  may  not  be  superfluous.  The  flower  of 
the  pea  with  its  standard,  wings,  and  median  keel  is  too 
familiar  to  need  description.  Like  most  flowers  it  is 
hermaphrodite.  Both  male  and  female  organs  occur  on 
the  same  flower,  and  are  covered  by  the  keel.  The  an- 
thers, ten  in  number,  are  arranged  in  a  circle  round  the 
pistil.  As  soon  as  they  are  ripe  they  burst  and  shed 
their  pollen  on  the  style.  The  pollen  tubes  then  pene- 
trate the  stigma,  pass  down  the  style,  and  eventually 
reach  the  ovules  in  the  lower  part  of  the  pistil.  Fertilisa- 
tion occurs  here.  Each  ovule,  which  is  reached  by  a 
pollen  tube,  swells  up  and  becomes  a  seed.  At  the  same 
time  the  fused  carpels  enclosing  the  ovules  enlarge  to 
form  the  pod.  When  this,  the  normal  mode  of  fertilisa- 
tion, takes  place,  the  flower  is  said  to  be  selfed. 

In  crossing,  it  is  necessary  to  emasculate  a  flower  on 
the  plant  chosen  to  be  the  female  parent.  For  this  pur- 
pose a  young  flower  must  be  taken  in  which  the  anthers 
have  not  yet  burst.  The  keel  is  depressed,  and  the  sta- 
mens bearing  the  anthers  are  removed  at  their  base  by  a 

187 


i88  MENDELISM 

pair  of  fine  forceps.  It  will  probably  be  found  necessary 
to  tear  the  keel  slightly  in  order  to  do  this.  The  pistil  is 
then  covered  up  again  with  the  keel,  and  the  flower  is 
enclosed  in  a  bag  of  waxed  paper  until  the  following  day. 
The  stigma  is  then  again  exposed  and  dusted  with  ripe 
pollen  from  a  flower  of  the  plant  selected  as  the  male 
parent.  This  done,  the  keel  is  replaced,  and  the  flower 
again  enclosed  in  its  bag  to  protect  it  from  the  possible 
attentions  of  insects  until  it  has  set  seed.  The  bag  may 
be  removed  in  about  a  week  after  fertilisation.  It  is 
perhaps  hardly  necessary  to  add  that  strict  biological 
cleanliness  must  be  exercised  during  the  fertilising  opera- 
tions. This  is  readily  attained  by  sterilising  fingers  and 
forceps  with  a  little  strong  spirit  before  each  operation, 
thereby  ensuring  the  death  of  any  foreign  pollen  grains 
which  may  be  present. 

The  above  method  applies  also  to  sweet  peas,  with 
these  slight  modifications.  As  the  anthers  ripen  relatively 
sooner  in  this  species,  emasculation  must  be  performed  at 
a  rather  earlier  stage.  It  is  generally  safe  to  choose  a  bud 
about  three  parts  grown.  The  interval  between  emas- 
culation and  fertilisation  must  be  rather  longer.  Two 
to  three  days  is  generally  sufficient.  Further,  the  sweet 
pea  is  visited  by  the  leaf-cutter  bee,  Megachile,  which, 
unlike  the  honey  bee,  is  able  to  depress  the  keel  and 
gather  pollen.  If  the  presence  of  this  insect  is  suspected, 
it  is  desirable  to  guard  against  the  risk  of  admixture  of 


APPENDIX  189 

foreign  pollen  by  selecting  for  pollinating  purposes  a 
flower  which  has  not  quite  opened.  If  the  standard  is 
not  erected,  it  is  unlikely  to  have  been  visited  by  Mega- 
chile.  Lastly,  it  not  infrequently  happens  that  the 
.little  beetle  Meligethes  is  found  inside  the  keel.  Such 
flowers  should  be  rejected  for  crossing  purposes. 


INDEX 


Abraxas  grossulariata,  gg 

"Acquired"  characters,  14 

Adaptation,  143 

Agouti  mice,  50 

Albino  mice,  50 

Albinos,  nature  of,  53 

Amauris,  144 

Analysis  of  types,  156 

Ancestral  Heredity,  Law  of,  13 

Andalusian  fowls,  70 

Axil  colour  in  sweet  peas,  g3 

Bateson,  W.,  14,  2g,  55,  116,  132,  141 

Biffen,  R.  H.,  157 

Blue  Andalusian  fowls,  71 

Brachydactyly,  171 

Bryony,  120 

Bush  sweet  peas,  63 

Castle,  132 

Cattle,  horns  in,  86,  166 

Colour,  nature  of,  in  flowers,  48 

Colour-blindness,  117 

Combs  of  fowls,  33,  43 

Correns,  C.,  2g,  120 

Coupling    of    characters    in    gametes, 

93 

Cuenot,  50,  ng 
"Cupid"  sweet  peas,  62 
Currant  moth,  gg 

Darwin,  C.,  10,  65,  147,  163 
De  Vries,  H.,  15,  2g,  141 
Discontinuity  in  variation,  14 
Dominant  characters,  18 
Doncaster,  L.,  gg 
Drinkwater,  H.,  172 


Dutch  rabbits,  60 

Eggs,  2 

Environment,  influence  of,  137 

Euralia,  144 

Evolution,  10,  85,  i3g 

Eye,  in  primulas,  55 

Eye-colour,  in  man,  176 

Factor,  definition  of,  31 

Factors,  interaction  of,  42 

Fertilisation,  3 

Fertilisation,  self-  and  cross-,  163 

Fixation  of  varieties,  153 

Fluctuations,  138 

Fowls,  coloured  from  whites,  4g,  73 

Gal  ton,  13,  i7g 
Gametes,  nature  of,  6 
Gregory,  R.  P.,  55,  g3 

Haemophilia,  176 

Hardy,  G.  H.,  147 

Heterozygote,  definition  of,  28 

Heterozygote,  of  intermediate  form,  68 

Hieracium,  27,  132 

Himalayan  rabbits,  60 

Homostyle  primulas,  56 

Homozygote,  definition  of,  28 

Hooded  sweet  peas,  8g 

Horses,  bay  and  chestnut  in,  167 

Hurst,  C.  C.,  62,  176,  180 

Immunity  in  wheat,  158 
Individuality,  135 
Inhibition,  factors  for,  74,  108 
Intermediates,  125 


191 


MENDELISM 


Johannsen,  W.,  160 
Lop-eared  rabbits,  132 

Mendel,  8,  17,  26,  132 
Mental  characters,  180 
Mice,  inheritance  of  coat  colour  in, 

50 

Mimicry,  143 
Mirabilis,  151 
Morgan,  T.  H.,  116 
Mulattos,  i2g 
Mutation,  83,  138 

Nageli,  C.,  26 

Natural  selection,  n,  140,  142,  149 
Nettleship,  E.,  175 
Night-blindness,  175 

Pararge  egeria,  132 

Parkinson,  J.,  122 

Pea  comb,  33 

Peas,  coloured  flowers  in,  24 

Peas,  tall  and  dwarf,  18 

Pigeons,  86 

Pin-eye  in  primulas,  55 

Pisum,  17 

Primulas,  31,  55,  68,  93 

Pollen,  3 

Pollen  of  sweet  peas,  92 

Pomace  fly,  115 

Population,   inheritance  of  characters 

in  a,  147 

Presence  and  Absence  theory,  35 
Pure  lines,  162 
Purity  of  gametes,  24 
Purity  of  type,  155 

Rabbits,  53,  60 
Ratios,  Mendelian  — 

3:1,  20 

9:3:3:1,  25,  34 

9:3:4,  Si 

9 : 7,  49 

Ray,  John,  143 
Recessive  characters,  19 
Repulsion  between  factors,  oo 


Reversion,  59,  165 

in  rabbits,  59 

in  sweet  peas,  62 

in  fowls,  65 

in  pigeons,  65 
Rose  comb,  33 

Saunders,  E.  R.,  54,  122 

Seeds,  nature  of,  4 

Segregation,  22  * 

Selection,  162 

Sheep,  horns  in,  76 

Silky  fowls,  30,  105 

Single  comb,  32 

Species,  nature  of,  150 

Species,  origin  of,  n 

Speckled  wood  butterfly,  132 

Spermatozoa,  3 

Sports,  147 

Staples-Browne,  R.,  66 

Sterility,  151 

Sterility  in  sweet  peas,  93 

Stocks,  double,  122 

Stocks,  hoariness  in,  54 

Sweet  pea,  colour  in,  44,  79 
history  of,  82 
inheritance  of  hood  in,  89 
inheritance  of  size  in,  62 

Telegony,  167 
Thrum-eye  in  primulas,  55 
Toe,  extra  toe  in  poultry,  76 
Tscherrhak,  E.,  29 

Unit-character,  definition  of,  31 
Variation,  14,  137,  139 

Walnut  comb,  33 
Weismann,  A.,  13 
Wheat,  beard  in,  74 

experiments  with,  157 
White,  dominant  in  poultry,  72 
Wilson,  J.,  1 68 

Yellow  mice,  119 
Zygotes,  nature  of,  5 


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OCT  21  1932 
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1933 


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1936 

HOV  1  1*  J937 
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APR  10  1939 
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NOV  13  1950 


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